Recent Gas Mileage

[scottmoyer]

Question regarding my recent gas mileage.

 

I was getting about 16.5 mpg with the factory tires (265/70-17) with my mixed driving. I replaced my tires recently with 285/70-17 but have not had my speedo or odo recalibrated yet. Based on calculations, I'm off 3 miles for every 70 or so. Here's my dilemna. Prior to the tire swap, I'd go 120 miles on the 1st 1/4 tank, 220 on the half and 300 at 1/4. Now, I'm at 180 at half. I've been trying to drive easier than before and have not seen any change. 180 miles with compensation for new tires is 188. I'm still down 32 miles at the 1/2 tank. My truck is also at 40k miles now. Is there more to the tire size change than just a simple calculation? The last fillup, the odometer read 286 and I filled up with 20.975 gallons. After tire compensation, that equates to 298 miles or 14.3 mpg!!! I do realize that the factory tires weighed 38# each and the new tires are 57# each, but that shouldn't affect my mileage this drastically, should it? Do I need some maintenance? Any thoughts?

[-Devil-]

that sounds about right to me... also need to take into account the extra weight of the air in the tires....

[scottmoyer]

The extra amount of air is negligible. I have about 10 psi more air than the old tires. The good thing is that the 10 psi doesn't equal 10#.

[99silverado1500]

Bigger tires = Less MPG no matter how you cut it. Weigh more and more effort required to spin them. If you go back to a 265 the MPG will be back up again.

[Paul58]

Bigger tires = Less MPG no matter how you cut it. Weigh more and more effort required to spin them. If you go back to a 265 the MPG will be back up again.

 

That really depends... Normally true for off-road tires because the more aggressive tread and wider footprint equate to higher rolling resistance. I swapped my stock 245/70-17's for 255/75-17 street treads and my mileage didn't change (if anything it got better, because I'm getting 17.8 mpg on my daily commute). The taller tire actually changed my final drive ratio from 3.73 to 3.55, so I'm rolling at a lower RPM at 70 mph than I was with my stock tires, and being as they are about the same width as the original, but with a bit more effecient tread pattern, mine's increase by about .1 to .2 mpg.

[Zembonez]

Every truck I have owned got less MPG with larger tires. Usually a 2 or so MPG loss. I'm sure it is due to the extra mass of larger tires / wheels plus the resulting change in final drive ratio. Worth it to me but I guess it might not be to everybody.

[diehrd]

Rolling weight which changed a lot will cause a loss of mPG ,, get a computer tune and you will rgain most if not all loss of mpg..wait4meperfomance has great deals on tunning

[cc1999]

That sounds about right to me as well. More unsprung weight = more fuel required to turn them. I am not shocked by those numbers at all. I went from 265/75/16 E (10 ply ) oem tires on my last diesel Excursion to a 295s and lost 3-4 mpg. It was a diesel, I was in hopes that all that torque would keep the loss to a minimum, nope it seemed to drop just as bad as some of the gas trucks that I had gone with big tires on.