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Tire Pressure


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53 replies to this topic

#21 OFFLINE   Don

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Posted 27 June 2013 - 11:30 AM

No company selling cars, into a market of high MPG cars would prioritize, ride comfort and "steering response" over MPG. The hybrid market is mostly about bragging rights for best MPG not comfort, luxury. or handling. Ford, like most other modern day companies, performs DFE testing to determine the best operating parameters of the vehicle. The recommended pressure is determined purely to optimize the safety of the vehicle. When NHTSA says they are not aware of hazards due to over inflation they obviously chose to close their eyes on the issue. There are many studies that show overinflated tires are more prone to blowouts when hitting road hazards such as potholes, etc.

 

Overinflating to save gas, I and Popular Mechanics thinks not: http://www.popularme...rs/news/4199963


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#22 OFFLINE   GaryG

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Posted 27 June 2013 - 03:08 PM

What is proper pressure?

The proper tire pressure for the Police Crown Victoria is 44 psi. If you look on the sidewall of the tire, you will see that it lists 44 psi max pressure. Regardless of what vehicle you have, use the maximum pressure listed on the sidewall. Higher pressure results in better performance, decreased tire wear, and it lessens your chance of hydroplaning at a given speed. This number on the sidewall lists “the maximum amount of pressure you should ever put in the tire under normal driving conditions.” Pursuits and Code 3 responses are not “normal driving conditions.” Many agencies maintain tire pressure at 35 psi since this is what is listed in the owner’s manual and on the door placard. The reason the owner’s manual lists 35 psi is because we get the same manual as the civilian version of the Crown Victoria. The police version, however, is fully loaded with communications equipment, a cage, and your gear. You are not looking for a soft and cushy ride, you want performance.

[b]Myths about pressure

Let’s put to rest some common misconceptions. The tires will not balloon out creating a peak in the center portion of the tread when tire pressure is above 35 psi. There is a steel belt that prevents this from happening. Also, you are not overstressing the tire with higher pressure, and the tire will not be forced off the rim with higher pressure. The picture above is Bobby Ore of Bobby Ore Motorsports driving a Ford Ranger on two wheels. The tires on the left side have 100 psi in them, and they happen to be tires and rims from a 1999 Crown Victoria! This is a dramatic example of how pressure holds the tire in shape, and how much stress a tire can handle.

Performance

If you were able to watch a tire as it travels across the ground at high speed, you would see that it deflects to one side during cornering. The faster you are going through a corner, the more tire deflection you get. As the tire deflects over onto the sidewall, you get less traction and more of a tendency to understeer or oversteer. This could spell disaster when negotiating a corner at high speed during a pursuit or a Code 3 run. Higher pressure keeps the tire from deflecting onto the sidewall as much, which keeps more of the treaded portion on the road.

A good demonstration for EVOC instructors is to have students drive a high-speed course in a vehicle with 32 to 35 psi. Then have them run the same course with 44 to 50 psi in the tires. The student will experience a marked difference in performance. Having officers experience this difference in vehicle performance is much more effective than just telling them to check their tire pressure.

Hydroplaning

When a tire rolls across a road covered with water, the tire tread channels water away so the rubber remains in contact with the road. The factors that affect hydroplaning are speed, and water depth. Conventional wisdom says that vehicles will hydroplane in as little as 1/16th of an inch of water. Not so coincidentally, legal tread depth is 1/16th of an inch.

Tire manufactures and the Association of Law Enforcement Emergency Response Trainers International (ALERT) have shown that tires have more of a tendency to hydroplane when pressure is low. This happens because the tire footprint (the portion of the tire actually in contact with the road) is larger. For those of you who water ski, think of which is easier to get up on: a fat ski or a skinny ski. More tire surface in contact with the water makes it easier to hydroplane, just as it is easier to water ski on a fat ski. Also, a soft tire can be pushed in more by the pressure of the water on the center portion of the tread. This results in less rubber in contact with the road.

Tire wear

Much better tire wear results from maintaining proper pressure. Tires with lower pressure will wear off the outside of the tread faster from the deflection of the tire during cornering, and the tires will heat up more from increased road friction. This is one of the factors that caused the failure of a certain brand of tires on Ford Explorers some years ago. In 1999 the San Jose Police Department realized a significant cost savings by increasing the pressure in the training fleet to 50 psi. They soon followed up by increasing the pressure in the patrol fleet to 44 psi. For liability reasons, most agencies are reluctant to exceed the maximum pressure listed on the tire for actual patrol vehicles, but they reap the cost saving when going to 50 psi on training vehicles.

Next time you inspect your vehicle, make sure you check your tire pressure since your ability to performance drive is significantly affected by it. You are not driving to the store to get a loaf of bread! You may be called upon to chase a dangerous criminal or respond to assist another officer in trouble. You don’t wonder whether or not your gun is loaded before you hit the street; don’t’ wonder whether your tire pressure is correct once the pursuit starts. Check your tires routinely, just as you do with all other critical equipment... http://www.officer.c...essure/19$27281

http://www.cleanmpg....hp/t-11652.html

 

I agree with this article because it matches with my and many other's experience.

 

Gary


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#23 OFFLINE   Don

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Posted 27 June 2013 - 03:53 PM

You continue to post only allegations and links to posts that also make allegations. The Popular Mechanics article documents an actual test with an actual vehicle with actual numbers of the tire pressures used and actual MPG's achieved during the experiment. The result was that increasing tire pressure did not increase MPG. Do you have any experimental data, from a credible source, that documents an actual experiment that validates increasing tire pressure, above manufacturers recommended pressure, increases the MPG. The information is no good unless all parameters are documented so they can be repeated by independent experimenters for validation.



#24 OFFLINE   altabrig

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Posted 27 June 2013 - 04:05 PM

Thank your for the education on tire pressure Gary and Dr. 61 - great posts.  I feel even better with 50 psi of N.

 

Based on driving on tires with higher psi in several different cars, the higher psi does seem to make for a harsher less predictable ride.   I can't comment on blowouts and hopefully it will stay that way.
 


Edited by altabrig, 27 June 2013 - 04:20 PM.


#25 OFFLINE   GaryG

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Posted 27 June 2013 - 04:23 PM

You continue to post only allegations and links to posts that also make allegations. The Popular Mechanics article documents an actual test with an actual vehicle with actual numbers of the tire pressures used and actual MPG's achieved during the experiment. The result was that increasing tire pressure did not increase MPG. Do you have any experimental data, from a credible source, that documents an actual experiment that validates increasing tire pressure, above manufacturers recommended pressure, increases the MPG. The information is no good unless all parameters are documented so they can be repeated by independent experimenters for validation.

 

Your response is non-sense again. Nothing in that article says anything about MPG.

 

Gary



#26 OFFLINE   dr61

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Posted 27 June 2013 - 05:13 PM

I suggest we focus on real data, not anecdotal information.  The NHTSA has real data, and they decided against high pressure warnings even though it could be done with the same TPMS equipment as low pressure warnings.  The Popular Science article is just a good story, not an engineering study.  There is no replication, they started more than 800' lower in elevation on the first leg (with higher pressure), and didn't take into account prevailing winds.  On the other hand, I don't see much in the way of replicated studies that show significant gains in fuel economy with tire pressures higher than the placard pressures.  Perhaps some of you could find some links?


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#27 OFFLINE   Don

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Posted 27 June 2013 - 05:37 PM

The PM article was not passed off to be an engineering study. It did however claim to debunk the myth that higher pressures increase fuel economy which is the same as saying improved MPG. The fact that they did include the details of their trial, to allow careful scrutiny (yes, I also realized the two legs of the trip were not the same), was much more useful to me than any of the anecdotal information presented so far. The article clearly states 41.19 MPG for one leg and 42.19 MPG for the other (Gary is in denial, is blowing smoke, or hasn't read the article).  Right or wrong at least PM shared the conditions that led to their conclusion which is a step in the right direction. I disagree about the NHTSA having real data to help them decide against high pressure warnings; they simply say they are not aware of any significant safety risks but they do not point to any experiments or data that shows they studied the overpressure condition. The NHTSA actually recommends that you run your tires within 4 PSI of the manufactures recommendation and they do not specify over or under.


Edited by Don, 27 June 2013 - 05:38 PM.


#28 OFFLINE   jeffegg2

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Posted 27 June 2013 - 05:45 PM

Anyone that has ever rode a bicycle with low air in the tires knows what the effect of tire pressure is. Simple test really.... Saying that tire pressure has no effect on MPG is just silly.



#29 OFFLINE   GaryG

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Posted 27 June 2013 - 06:17 PM

Anyone that has ever rode a bicycle with low air in the tires knows what the effect of tire pressure is. Simple test really.... Saying that tire pressure has no effect on MPG is just silly.

 

Did you read this Don? I agree, you are just silly.

 

Gary



#30 OFFLINE   Don

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Posted 27 June 2013 - 06:49 PM

Again with the anecdotes. Where is the data with numbers? Riding a bike on a flat is indeed more difficult than a riding a bike with fully inflated tires. Keep inflating those tires say 10 PSI at a time and report back on how much easier it gets (report in some energy unit expenditure) and see what you get. Don't forget to keep inflating until the tire pops. Find out the failure pressure (you will have to destroy a tire to determine this) then work your way back down say 10 PSI at a time and take your bike through typical road conditions found in everyday life (smooth asphalt, loose gravel, concrete with roughness and maybe a rock or two). You will then have the data needed to help determine the proper pressure. I never said that tire pressure has no effect on MPG. What PM is saying is that increasing the tire pressure, over the car manufacturers recommended tire pressure, does not improve MPG. I would modify their statement and say there is no significant improvement in MPG when inflating over the car manufactures rating. PM did crude trial to support their statement but at least they provided real data and described the test conditions. What is silly is that you really don't seem to understand what is written.



#31 OFFLINE   smangerer

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Posted 27 June 2013 - 09:59 PM

This debate is truly entertaining and I thank you all for the read on this and other threads. A few comments if I may;

 

I would guess that they didn't choose to warn for over-inflating because very few people would ever go that far over recommendation. Underinflation however can occur on its own so it must be monitored.

 

Now about pressure .. I havent yet added any air or checked my cars tires since its so new but I have given my tires a small bump over recommendation with past vehicles to cover loss of pressure over time. I also would like to add that if you do decide that pumping your tires up to the maximum pressure .. please account for elevation changes. Here in CA I can go from 0 feet to over 7000 in a few hours and I do it often. I would rather use close to a recommended number considering I may have my child in my car. I dont care about getting an alleged few decimal points better MPG and certianly dont plan on any high speed pursuits (You know that driving like that kills MPG ;) ) when the safety of my little girl is considered. 

 

To be fair... yes an under-inflated bike tire sucks but we are not talking about under-inflating our tires but rather to fully inflate them to the recommended preasure for the car. Also when speaking of Car tires, you are talking of sigifigant temperature changes due to durration and speed that bikes never see. Bike tires just dont compare. I get it but it isn't a good comparison. 


Edited by smangerer, 27 June 2013 - 10:04 PM.


#32 OFFLINE   Don

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Posted 28 June 2013 - 06:26 AM

 but rather to fully inflate them to the recommended preasure for the car. Also when speaking of Car tires, you are talking of sigifigant temperature changes due to durration and speed that bikes never see. Bike tires just dont compare. I get it but it isn't a good comparison. 

 Actually the discussion is about inflating the tire to the max pressure rated on the sidewall of the tire which is many PSI greater than the car manufacture's recommended pressure for the car. Like I said before when a car manufacture puts a product into the high MPG market they would like nothing more than to claim the highest MPG possible. If it were as simple as putting more air in the tires they would do it in a heartbeat; ask yourself why they don't do it. Gary posted, in another thread on this subject, the reason for the lower recommended pressure is a conspiracy between car companies and tire companies to sell more tires. Who is being silly?



#33 OFFLINE   smangerer

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Posted 28 June 2013 - 08:57 AM

I get it. I was replying to the comparison of under-inflated bike tires to a recommended inflated car tire. The point that Jeff was trying to make was that an under-inflated tire isn't as productive and I reminded that No one on this discussion is talking about under-inflating their car tires but as you have stated, to not max them out.



#34 OFFLINE   timwil56

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Posted 14 March 2015 - 03:39 AM

I did a Google search for Tire Over Inflation and searched the first 10 pages. There have been some links posted here and in another thread regarding the benefits of over inflation to the maximum pressure listed on the sidewall, but I could not find any articles in my search that recommended over inflation beyond 10% of the pressure rating from the car manufacturer. I didn't include various University studies on automobile and truck tire inflation because they were very wordy and a bit too academic. In one of the articles below, it states a 10% increase in pressure is acceptable so, 1.10 x 38 = 41.8 psi, rounded down for safety equals 41 psi, which the author believes is a safe overinflation pressure, and I agree with that. I did not include the tire manufacturer links, because not one recommends over inflation.
 

http://www.popularme...s/a940/4199963/

http://www.hybridcar...Inflating-Tires

http://auto.howstuff...t-tire-wear.htm



#35 OFFLINE   drdiesel1

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Posted 14 March 2015 - 08:21 AM

No body will ever make that statement. Tire pressure liability has become a hot topic ever since the Ford/Firestone

fiasco. Today's way of doing business has everyone sue happy and most states have adopted the vehicle tire label

as the bible for setting requirements. CA has a law that any vehicle that comes into a repair facility for service work

is required to have the tire pressure set to that spec. The only way around it is to have the customer sign a waiver

and keep it on file with his service records. I run a higher PSI to get better MPG's, tire wear and better handling.

 

The CA CHP and San Jose PD both did a study on higher tire pressures and they have both concluded it's better

to run higher pressures for the same reasons, I stated above. Overall tire life and better handling were the key improvements

for them and better MPG numbers will be an added benefit for us. Following the "RECOMMENDED" pressure is okay

if you like to drive a bowling ball. It squirms to much for me. It's ill handling and could easily cause a dangerous situation

in an emergency maneuver, IMO.  I had this happen with my new truck. I had to swerve to avoid another vehicle and it

started to roll the tires as it began to lean @ 60 mph. My truck weights 7400 lbs. That was @ the door label spec

and it's too low. I raised them up 15 psi and it made a night and day difference. Never felt so out of control in a vehicle

as I did that day. Having a Ram Diesel 4x4 crew cab swaying around in traffic isn't fun.

 

Upping tire pressure is adding safety to you driving regardless of the many varied opinions you find on the Interweb :wink:

40% of the cars I see on the road all have low tires and abnormal wear from running them low. That's the real danger :victory:

Low tires run hotter. They will cause an over correction situation with loss of control in a emergency/panic situation :shift:


Edited by drdiesel1, 14 March 2015 - 08:23 AM.


#36 OFFLINE   engnrng

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Posted 14 March 2015 - 12:00 PM

No body will ever make that statement. Tire pressure liability has become a hot topic ever since the Ford/Firestone

fiasco. Today's way of doing business has everyone sue happy and most states have adopted the vehicle tire label

as the bible for setting requirements. CA has a law that any vehicle that comes into a repair facility for service work

is required to have the tire pressure set to that spec. The only way around it is to have the customer sign a waiver

and keep it on file with his service records. I run a higher PSI to get better MPG's, tire wear and better handling.

To clarify what drdiesel1 refers to, the Ford/Firestone fiasco was an underinflation problem, not overinflation.  FIrestone recommended a higher pressure to Ford than what Ford ultimately put on their door frames on their big SUV's, and my understanding is that tires failed due to too low pressure.  The State of California defines "significantly underinflated" as 25% under the vehicle manufacturer recommended values.

 

The California law and Air Resources Board Regulation both refer to avoiding underinflation, and the title of the regulation is "Subarticle 8.  Regulation for Under Inflated Vehicle Tires; §95550.  Regulation for Under Inflated Vehicle Tires".  Of course, if the vehicle manufacturer recommends 38 psi and the tires are at 44 psi, they are not underinflated!!!  Although the way the regulation is written is quite silly, other questions and answers have clarified it.  For example, the regulation states:

  • Check and inflate each vehicle’s tires to the recommended tire pressure rating, with air or nitrogen, as appropriate, at the time of performing any automotive maintenance or repair service.

"Question:  If the tire pressure is above the vehicle manufacturer recommended rating, will I be fined?  Answer:  No"  This was from a FAQ on a web site by the California Board of Equalization for licensed mechanics.  Another:  "The state of California has passed a new law beginning July 1, 2010 that will require nearly 40,000 automotive servicing shops to check tire pressure of vehicles during each visit and correct the tire pressure by adding air if necessary."  The law does not require removing air. 

 

Any mechanic or person with common sense knows that if you reduce tire pressure from 51 psi to 38 psi for example, that is tire DEFLATION, not tire INFLATION.  You cannot INFLATE a tire from 51 psi to 38 psi.  From the original regulation:  "The purpose of this regulation is to reduce greenhouse gas emissions from vehicles operating with under inflated tires by inflating them to the recommended tire pressure rating."  Obviously, and with certainty, the purpose of the regulation is NOT satisfied by mechanics REDUCING tire pressure from some higher value to the vehicle manufacturer recommended value.  There is no need to sign a waiver for higher pressures as long as the sidewall maximum is not exceeded.  Any mechanic who claims that requirement is ignorant of the law and its enforcement.  A waiver is required if the vehicle owner wants the pressure left BELOW the recommended setting.  No competent mechanic who finds a cold inflation tire pressure above the sidewall would leave it there, but only needs to reduce the overinflation down to the sidewall maximum. 

 

Many owner manuals (especially pickup trucks) state that more air pressure may be needed above the door frame posted recommended pressure if a load is carried, but do not exceed the maximum load and pressure on the tire sidewall.  I have educated my mechanic for my other vehicles about the wording of the California law, and am now working on my local Ford Dealer mechanics.  The tire pressures listed on the door frame are a minimum tire pressure, not a maximum.  The tire pressure maximum is molded into the tire sidewall.  Any articles on overinflation are typically referring to exceeding the maximum rating on the sidewall.  A tire is not overinflated until the cold inflation pressure is higher than the sidewall maximum.

 

The sidewall pressure and the door frame recommended pressures are cold inflation pressures. On my C-MAX, any cold inflation tire pressure between 38 psi and 51 psi is legal and safe. 


Edited by engnrng, 14 March 2015 - 12:15 PM.

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#37 OFFLINE   engnrng

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Posted 14 March 2015 - 12:23 PM


Now about pressure .. I havent yet added any air or checked my cars tires since its so new but I have given my tires a small bump over recommendation with past vehicles to cover loss of pressure over time. I also would like to add that if you do decide that pumping your tires up to the maximum pressure .. please account for elevation changes. Here in CA I can go from 0 feet to over 7000 in a few hours and I do it often. I would rather use close to a recommended number considering I may have my child in my car. I dont care about getting an alleged few decimal points better MPG and certianly dont plan on any high speed pursuits (You know that driving like that kills MPG ;) ) when the safety of my little girl is considered. 

 

 

 

From sea level to 7000 feet is an ambient air pressure change of about 3 psi. 



#38 OFFLINE   timwil56

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Posted 14 March 2015 - 04:04 PM

OK, so let's say you leave in the morning with the tires cold at 51 psi and the air temp at 60 degrees. You take the long way and eventually drive to 7000 feet where the pressure increases 3 psi, add another 2-3 psi for heat from friction and 3 psi for a 20 degree increase in ambient temperature from AM to PM, now your tire pressure is at 60 psi, 12 pounds above what Ford recommends and 9 pounds above the tire manufacturer’s maximum rating. You better not hit that pot hole or have to make a moderate to high speed emergency maneuver. But hey, you saved a half mile of battery energy or .10 gallons of gas, so why wouldn't you do it.


Edited by timwil56, 14 March 2015 - 04:05 PM.


#39 OFFLINE   dr61

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Posted 14 March 2015 - 05:19 PM

The tire makers have thought through all of these scenarios.  Sea level pressure, 68F; you will be fine at any pressure from the door placard recommendation to the tire sidewall maximum.  The only real problem is very low temperature which causes loss of pressure.

 

I personally set my C-Max tires at 43 PSI at 68F, near sea level.  Not because I think higher pressure is dangerous, but because I like the combination of ride, handling, and rolling resistance on the usual pavement I drive on.  If it gets 20 F colder, I inflate to 43 PSI.  I use my racing calibrated tire gauge and my compressor.



#40 OFFLINE   drdiesel1

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Posted 14 March 2015 - 05:35 PM

To clarify what drdiesel1 refers to, the Ford/Firestone fiasco was an underinflation problem, not overinflation.  FIrestone recommended a higher pressure to Ford than what Ford ultimately put on their door frames on their big SUV's, and my understanding is that tires failed due to too low pressure.  The State of California defines "significantly underinflated" as 25% under the vehicle manufacturer recommended values.

 

The California law and Air Resources Board Regulation both refer to avoiding underinflation, and the title of the regulation is "Subarticle 8.  Regulation for Under Inflated Vehicle Tires; §95550.  Regulation for Under Inflated Vehicle Tires".  Of course, if the vehicle manufacturer recommends 38 psi and the tires are at 44 psi, they are not underinflated!!!  Although the way the regulation is written is quite silly, other questions and answers have clarified it.  For example, the regulation states:

  • Check and inflate each vehicle’s tires to the recommended tire pressure rating, with air or nitrogen, as appropriate, at the time of performing any automotive maintenance or repair service.

"Question:  If the tire pressure is above the vehicle manufacturer recommended rating, will I be fined?  Answer:  No"  This was from a FAQ on a web site by the California Board of Equalization for licensed mechanics.  Another:  "The state of California has passed a new law beginning July 1, 2010 that will require nearly 40,000 automotive servicing shops to check tire pressure of vehicles during each visit and correct the tire pressure by adding air if necessary."  The law does not require removing air. 

 

Any mechanic or person with common sense knows that if you reduce tire pressure from 51 psi to 38 psi for example, that is tire DEFLATION, not tire INFLATION.  You cannot INFLATE a tire from 51 psi to 38 psi.  From the original regulation:  "The purpose of this regulation is to reduce greenhouse gas emissions from vehicles operating with under inflated tires by inflating them to the recommended tire pressure rating."  Obviously, and with certainty, the purpose of the regulation is NOT satisfied by mechanics REDUCING tire pressure from some higher value to the vehicle manufacturer recommended value.  There is no need to sign a waiver for higher pressures as long as the sidewall maximum is not exceeded.  Any mechanic who claims that requirement is ignorant of the law and its enforcement.  A waiver is required if the vehicle owner wants the pressure left BELOW the recommended setting.  No competent mechanic who finds a cold inflation tire pressure above the sidewall would leave it there, but only needs to reduce the overinflation down to the sidewall maximum. 

 

Many owner manuals (especially pickup trucks) state that more air pressure may be needed above the door frame posted recommended pressure if a load is carried, but do not exceed the maximum load and pressure on the tire sidewall.  I have educated my mechanic for my other vehicles about the wording of the California law, and am now working on my local Ford Dealer mechanics.  The tire pressures listed on the door frame are a minimum tire pressure, not a maximum.  The tire pressure maximum is molded into the tire sidewall.  Any articles on overinflation are typically referring to exceeding the maximum rating on the sidewall.  A tire is not overinflated until the cold inflation pressure is higher than the sidewall maximum.

 

The sidewall pressure and the door frame recommended pressures are cold inflation pressures. On my C-MAX, any cold inflation tire pressure between 38 psi and 51 psi is legal and safe. 

This clears up all the misinformation about the requirements and obligations quite nicely. Thanks for posting this :victory:










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