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Tire Pressure


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32 replies to this topic

#1 OFFLINE   jeffegg2

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Posted 10 February 2013 - 04:44 PM

Got about 1k miles now, decided to check the tire pressure and sure enough, only 40 pounds of pressure in tires rated at 51 pounds. Today I inflated them to the full 51 pounds.


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#2 OFFLINE   dr61

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Posted 11 February 2013 - 09:38 AM

Got about 1k miles now, decided to check the tire pressure and sure enough, only 40 pounds of pressure in tires rated at 51 pounds. Today I inflated them to the full 51 pounds.

What tire pressure does Ford recommend for this car? Ford's cold inflation pressure recommendation is on a plaque in the driver door jam area.  If I am not mistaken I think I have read that it is 38 psi for normal loads.  The 51 psi listed on the tires is the tire maker's maximum pressure for the tire's full load, and is not specific for this car.  If you choose to use 51 psi, ride quality may deteriorate a bit, rolling resistance may go down slightly, and traction may not be optimal, especially on rough roads. Transient steering response may improve slightly. In my extensive experience with tire pressures, the differences are most likely to be small, and not easily measured without instrumentation.



#3 OFFLINE   jeffegg2

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Posted 11 February 2013 - 04:24 PM

Not so much worried about "ride quality" i.e. "soft ride", as much as fuel economy. There is a safety margin built in so I could actually exceed the max by 10% and not be too worried myself, but the 51 seems fine.



#4 OFFLINE   dr61

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Posted 11 February 2013 - 06:03 PM

True, there is always a safety margin; I too have exceeded sidewall max when testing handling alterations for competition (autocross).  However there is unlikely to be significant reduction in rolling resistance by increasing tire pressures more than 25% above specified.  You might try some coast-down tests with OEM pressure vs. 51 psi to see if you can measure a difference.



#5 OFFLINE   Tdefny

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Posted 11 February 2013 - 07:56 PM

What about traction and tire wear. I would think Ford came upon their recommendations for a reason - especially if fuel economy was a high priority.

#6 OFFLINE   Deermouse

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Posted 11 February 2013 - 09:44 PM

From the owners manual:

 

Use the recommended cold inflation pressure for optimum tire

performance and wear. Under-inflation or over-inflation may cause

uneven treadwear patterns.

Always inflate your tires to the Ford recommended inflation pressure

even if it is less than the maximum inflation pressure information found

on the tire. The Ford recommended tire inflation pressure is found on

the Safety Compliance Certification Label (affixed to either the door

hinge pillar, door-latch post, or the door edge that meets the door-latch

post, next to the driver’s seating position), or Tire Label which is located

on the B-Pillar or the edge of the driver’s door. Failure to follow the tire

pressure recommendations can cause uneven treadwear patterns and

adversely affect the way your vehicle handles.

 

They don't go into any more detail than this.  I checked Click and Clack (Car Talk guys) and MotorWeek, but they did not give more detail than this either. But the concensus is consistent with Tdefny's comment - go with what the manufacturer suggests.

 

Given that it's part of a safety compliance label, I'm thinking I'll follow their recommendation.

 

 

 



#7 OFFLINE   astrand1

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Posted 11 February 2013 - 09:55 PM

Mine were set at 40 when I brought it home. I increased it just a little and set them all at 45psi.

#8 OFFLINE   pureenergi

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Posted 13 February 2013 - 07:49 AM

I went up to the max on the sidewall, just like I did on my last car for years with no ill effects.  If the tires wear more in the center (didn't have that problem on the last car, and these are wider, flatter tires in terms of contact patch) lower the pressure later on to compensate, and tire rotation will mitigate this somewhat too - different tires have different jobs to do.

 

The ride is comfy in this car, and the sidewalls are short for the weight of the car, and I want great fuel economy.  The tire deforms a lot even at 51psi when you put most of the car's weight on a curb - see below.  I wouldn't want to under inflate any car (that's more dangerous than overinflating) but I certainly wouldn't want to on this car.  Also, my MPG and MPGe are pretty good - about 88 and 57, I think - the former is more affected by plugging in often than anything else.

MPGe after a long 500+ mile trip followed by more of the usual around town commuting and errands - 56.6 MPGe
Still some tire deformation - heavy car, steep downhill curb, even at 51psi!


#9 OFFLINE   fwroberts

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Posted 04 April 2013 - 09:02 AM

Thanks all, this was very helpful! Just took my snow tires off and had them pump the tires up 40 psi. We'll see how that goes.

Rick

#10 OFFLINE   jeffegg2

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Posted 05 April 2013 - 08:46 AM

Someone still uses snow tires? I haven't seen that in years!

#11 OFFLINE   fwroberts

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Posted 05 April 2013 - 09:00 AM

Depends on where you live!

#12 OFFLINE   shaggy314

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Posted 05 April 2013 - 12:35 PM

yeah, here in Central Texas, I've NEVER seen snow tires...  ;-)



#13 OFFLINE   hljmesa

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Posted 20 June 2013 - 03:17 PM

The tires on the C-Max are 17's........Are they Firestone ???



#14 OFFLINE   dr61

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Posted 20 June 2013 - 03:40 PM

The tires on the C-Max are 17's........Are they Firestone ???

Michelin Energy Saver A/S 225/50-17.


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#15 OFFLINE   altabrig

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Posted 25 June 2013 - 11:28 AM

Running 50 PSI all around for the last two months.  LRR and MPGe is great  60.3 lifetime at over 6k miles.  Over 55 mph the car wants to dart a bit at the high pressure vs. the factory 38 psi that we ran for a couple of months.



#16 OFFLINE   Smiling Jack

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Posted 25 June 2013 - 01:25 PM

Running 50 PSI all around for the last two months.  LRR and MPGe is great  60.3 lifetime at over 6k miles.  Over 55 mph the car wants to dart a bit at the high pressure vs. the factory 38 psi that we ran for a couple of months.

 

to "dart" ?

 

Please explain.



#17 OFFLINE   HOV MAX

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Posted 26 June 2013 - 05:29 PM

Got about 1k miles now, decided to check the tire pressure and sure enough, only 40 pounds of pressure in tires rated at 51 pounds. Today I inflated them to the full 51 pounds.

 

 

At what point does the TPMS sensors kick in and tell you that the tires are under inflated?....



#18 OFFLINE   Don

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Posted 27 June 2013 - 06:18 AM

The rated pressure on the tire and the proper tire pressure on your car are different. Check your owners manual, or the sticker in the drivers side door jam, for the proper pressure for your C Max. When you inflate over the manufactures recommended pressure you are making the car less safe.



#19 OFFLINE   altabrig

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Posted 27 June 2013 - 07:47 AM

^ True.  The only reason to run 50 psi is if you are trying to eeek our every last bit of efficiency/MPGE.  The higher pressure LRR makes the car "quirkier" at highway speed.  It tends to dart (wander or track off course a bit with a slight bump in the road) and is more sensitive to steering input, but the harder tires roll easier at the expense of grip and stability.


Edited by altabrig, 27 June 2013 - 07:48 AM.


#20 OFFLINE   dr61

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Posted 27 June 2013 - 10:53 AM

Tire pressure settings are a very complex subject.  I hesitate to go into this in depth (I have a lot of experience in this field), but I have a few observations that may be helpful.

 

Modern radial passenger car tires generally have a wide range of pressures in which they perform about the same in instrumented tests of traction on various surfaces and wet/dry/snow conditions.  The major safety issue with tire pressure is under inflation, which causes heat buildup and potential tire failure.  This is the reason that tire monitoring systems are now required.  The NHTSA rule on tire pressure monitoring is here:

http://www.nhtsa.gov...inalrule.6.html

 

Note that in this ruling NHTSA declined to require monitoring of over-inflation (see sec. 15). They say: "Furthermore, we are not aware of vehicle safety data reporting over-inflated tires as a significant safety hazard. In addition, available information does not suggest that over-inflation has the same safety implications as under-inflation, which causes heat buildup in a tire, potentially leading to permanent tire damage and sudden failure."

 

Many European cars have two pressure settings on their door placard and manual.  My 2012 Mini has a setting for speeds 99 mph and below, and another setting for speeds 100+ mph.  They are 6 psi apart.  Obviously it is safe to drive with at least 6 psi higher than the low speed setting, as one will  have to drive from 0-100 mph to exceed 100 mph.

 

There are always compromises when the car company sets a single recommended tire pressure.  Several obvious ones are ride quality and steering response.  Ride comfort decreases and steering response increases with pressure (perhaps to the point where the steering may feel like it darts as altabrig points out).  On the other hand rolling resistance may decrease slightly up to a point.  Also at some point above the recommended pressure, lateral tire grip will start decreasing significantly.  In my experience that point is above 50 psi for most car/tire combinations.  Determining these limits takes instrumented tire testing.

 

I personally run my cold tire pressures about 4 psi high (42 psi in my garage, cold morning temperature with shade).  I check every month or before a long trip.  I like the slightly quicker steering response this gives, and I don't feel much ride deterioration on the streets we usually drive on.  I doubt I can tell if fuel efficiency is significantly better at this setting.

 

I think it is fine if people stay at the 38 PSI placard recommendation.  If you do, however, you should check that pressure frequently as they will go down with time. The monitoring system will probably not report unless the pressure is at least 10% low.  Setting the pressure a bit higher will give a cushion to keep the pressures at least at 38 psi.  My dealer sets the pressures at 40 psi for that reason.


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