This might sound dumb but im running 31s on my 89 pickup and i know my odometer turns slower that with the stock tire size, but is the a mathematic formula for figuring out my gas milage. I think my stock tire size is 225x75x15 so that makes them 28.3 inches. just wondering if any body knows

 

any body

 

Not sure if there is one, sorry can't help. Have you done a google search?

 

If your truck came with 225's, you had a tire circumference of 88.86" ( Pi x diameter). Your 31's have a circumference of 97.34".

Divide 97.34 by 88.86 and you get 1.09. If you take your odometer reading and multiply it by 1.09 you'll get your actual mileage.

So if your mileage on a tank says 250, you really went 272.5 miles (250 x 1.09 = 272.5)

 

man i wonder how i would know if my truck came with 31s or 28s, mabey im getting a little better milage

 

wow thanks a bunch guess i should have finished college huh

 

On-line calculator to figure out all the above:
- http://www.4crawler.com/4x4/4LoCalc.shtml

As far as 28" or 31" factory tires, look at your gearing:
- http://www.4crawler.com/4x4/4R_TechI...oDetermination

If you have 4.10 (M/T) or 4.3x (A/T) then your truck came w/ 28s, if it has 4.56 (M/T) or 4.88 (A/T) then your truck came with the factory 31" tire option.

 

well anytime you add bigger tires the speedo wil be off and that effects the odminder. you might have to have the speedo recalibraded

 

Similar: https://www.yotatech.com/forums/f5/a...ometer-110732/

 

I still haven't started....Probably should before I have kids, lol.

 

Look inside your door jam. It's got a tag that says what size tires you have.

Your speedo is off by 10%, so as was pointed out, add that to your mileage and you should be better at what you actually get.

Either that, or drop in a set of new gears on your differentials.

 

what sucks is when you cant remember what size were on it when you first got it, i remember i put 30s on it like 3 years ago, and last year i put 31 bfgs on it. lol. im GUESSING its actually around like 280,000

 

i just put on 265x75s on my 91 Surf and i have IMPROVED my fuel economy! try and explain that to me lol

 

you changed your final gear ratio.

As a result, your odometer is giving you a false reading until you correct for it mathematically.