I've read claims to the contrary but I'd bet that given strict controls you would see very little differences.
I'm not sure what you mean. I believe I'm gaining 2 mpg at 70 mph and it would diminish down to only a little at 35 mph. So the faster you go the more Grill Covers help Aerodynamically. On the Flip side the slower you go the more Grill Covers help to keep ICE WT up (operating temp 202-212*F) to make ICE more thermally efficient. At speeds less than 55mph the ICE WT never gets up to operating temps. I keep my Grill Covers on pretty much all the time. The center one never comes off. The Lower Grill Cover has 7 x 1.5" holes in lower Grill Cover towards Driver side in a row.with Velcro-ed LEXAN cover over it. Taking it off lowers WT about 7*F, taking lower Grill Cover off lowers it 15*F.