IAJack

Since I had new Cooper AT3's put on my fuel milage has dropped a bit. I was getting 21 mpg on the hwy with the 4.6 on the Hankook Dynapro 235/70/17's. I am now down to 18.9 mpg hwy.

Last week I has the AT3's put on in 265/70/17's

It is hard to tell for sure what is the cause but it could be a couple things. They may be switching over to winter blend fuel here in Iowa / Midwest so that could be partially to blame?

The AT3 specs seem to say that they are 41lbs each which would be about 8 lbs heavier per tire. I am assuming this is most of the cause.

It has been cooler here the last couple weeks with the avg high around 74 degrees.

I always seem to get better mpg with 91 octane? I have read where that isn't supposed to make a difference but it has always seems to bump it up a few points with my truck.

Any thoughts and dos the extra tire size account for this or do I possibly need to look at a tune up?

brihvac

I would love to see that mileage. I have never seen over 16mpg. Its usually 14 and I baby it.

RLXXI

The larger (heavier) tires and increased rolling resistance are what's killing the fuel economy.

You bought a truck, economy should be the last thing on your mind.
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IAJack

I am not really worried about it and the 18.9 mpg is better than I expected to get out this truck from the beginning.

theduke308

You also went from 235's to 265's, so I would bet you speedometer is now off 3-5 MPH as well, this happened on my 04 when I went from stock 235's to 265's, that will make your odometer slightly incorrect as well and will make your fuel mileage look worse than it actually is

IAJack

According to my GPS my spedo is right on? Does different tire size make a difference to the computer even f the spedo is correct?

TJPlatinumEB

Does your truck still have the sticker on the driver side door indicating what size tires were on it from the factory? I only ask because you may have actually "upgraded" to a near stock size tire with the new size you went with, thus the reason the speedo is accurate.

You went from a 29.952" tall tire to a 31.606" tall tire. That alone would make a difference on the speedo. If it is accurate now, then it probably was not accurate before the tire change.

blderman

I lost about .5-1mpg when I switched to 285/70/18 Cooper AT3s. I can still get 20+mpg HWY as long as there isn't any headwind. In town is where the bigger mpg hit was noticed.

theduke308

blderman, did you chance sizes when you got the At3's?

TuxBlackFX4

Yes, larger, heavier tires are hurting your mileage. I disagree with this statement. You have a truck, you pay for gas. You don't like paying more for gas than you have to. So I don't see why it should be the "last thing" on your mind.