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Can I fit Nn size tires on my truck? What MPG will I get?

2K views 5 replies 3 participants last post by  KevinK 
#1 · (Edited)
Can I fit NNxN-NN or nnn/nn-nn size tires on my truck?


Yes, but what are you willing to do to make them fit?

Some mythbusting about big tires, especially 37s…


You need at least a 6 inch lift to run 37s

Not true

If you are willing to do an aggressive body mount chop, trim the skid plate and trim the stock bumper, you can run 37s on stock upper and lower control arms with about 2” of lift in the front and 1” lift in the rear.


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


Offset, Offset, Offset

Fitting bigger tires involves way more than whether or not you can physically bolt them on and roll in a straight line. Everyone wants that badass look of ‘tire poke’ because it makes the truck look mean and stuff. It’s true, and there’s a reason we associate tire poke with badassery – serious off-road (and on-road) racecars run ludicrously wide tires for good traction. Big fat tires stick out. Those cars and trucks go fast and look cool. We want to go fast and look cool. We need tire poke. Ludicrously fat tires have directly relative price tags and inversely proportional availability – they’re hard to find and cost a shitload.

Simple solution is to get rims with really low offset so they poke out past the fenders and look cool. We did this on our first set of rims and tires. We got +10 offset on an 8” rim and they looked badass.

We could not run +10 offset on the tires we have now. The arc described by the outside of the wheel as it swings through its steering cycle from lock to lock is increased the less offset you have. To illustrate this, let’s exaggerate what happens. Imagine you put a 2 foot spacer on the wheel to make it sick out about 2 feet past the fender. It would look ridiculous, but more importantly, the tire would crash into the door or the outside of the front bumper if you tried to turn. What happens with low offset rims (wheels that stick out) is the tire bumps into the body mount and the pinch weld and the skid plate and the bumper when you run bigger tires. To get the tire sucked in to where it doesn’t hit as much metal, you have to run a higher offset (less backspace) with less poke.

We have 18x9 with +25 offset, and while we did need to trim some metal, it’s not anything you can see from outside of the wheel well.






I thought I had more to say, but I’ll post this now and think of more later.


Now I remember...

Uptravel

When we installed our BOSS front coilovers, they came with bump stop spacers to reduce the amount of uptravel the front suspension can cycle through. This prevents the shock itself from becoming the limiting factor and blowing out the shock. It has the added benefit of reducing the probability of the top of the tire hitting the top of the wheel well.
 
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