I'll quote myself from another post I made on here after I put 37s on.
Running huge tires will destroy gas mileage
Meh, not really
Not to burst everyone’s bubble, but the actual number of miles your truck travels per every gallon of fuel burned is probably not significantly worse with larger tires. The reason your miles per gallon looks terrible is because the number of rotations per mile hasn’t been corrected. If you’ve corrected your speedometer, your mpg computer MAY be reading about right (depends on your solution to fixing the speedo), but here is the typical reaction:
“I put a lift and bigger tires on my truck and now I get terrible gas mileage! Eff you Toyota! Wah wah wah!”
When we replaced our 33s with 37s, our mileage did indeed drop to about 12 mpg from about 15mpg. “Oh my God, the sky is falling!!! How will we ever afford steak again?!?!” To see if we could still eat like royalty (or at least not have to survive on cat food) I did the maths…
Let’s look closer at those last three columns – they’re the important ones…
Basically, the truck thinks it has driven a mile for every 628.3 rotations of the tires (this is based on the stock tire size of 275/65-18 for a 2014 DC 4x4 SR5). 628.3 rotations of a 37” tire takes you farther than 628.3 rotations of a 32.1” tire. 1.15265 miles to be almost exact.
So, if I take the 12 mpg my truck says I’m getting, and multiply that by the 1.1526 miles we actually travel for every mile the truck ‘thinks’ we drove, I’m actually getting 13.8 miles per gallon – that is only 1.2 mpg worse than stock tires – on 37s (13.5 inches wide too, not 12.5). If I believed my trip computer I would think I lost 3 mpg, but really only lost 1.2.
Math is fun kids.
Here’s a cheat sheet for anyone interested but lazy. To find your actual mpg, take what the computer says (or you calculate by hand) and multiply it according to this list:
For 33” tires, multiply by 1.03
For 35” tires, multiply by 1.09
For 37” tires, multiply by 1.15
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I should note here that I'm not implying that you are whining about mpg as I implied above - I just quoted myself about general reactions to bad mpg due to bigger tires