i just want to know how to figure out my actual mpg. so i got 220.2 mi on 18.2 gal. with just that i got 12 mpg, but i want to know how to adjust for running 31" tires. i have a 4.0, aw4 and 3.55.

any help would be great.

So I'm assuming your speedometer is way the hell off?

Alright I got the exact problem you have. Same deal, 31s, stock alloy wheels and new exhaust and cat and intake blah blah blah

And still can only get 12 mpg. I just got the new speedo gear to install. Hoping it calculates better

i know theres a way to figure it out, and just hoping someone one here is smart enough to help me out.

I am running 35's and had the same problem. but with the electronic odometer it's pretty easy on the high way stop next to a milemarker and take note of your odometer then go to the next mile marker and see what you have or for a more precise calculation go ten or twenty miles then see how far your odometer has ran I calculated that with 35's my odometer is off .2 miles for every mile I actualy drive so with 31's this number will be smaller but once you have figured that out just multiply the number of miles you have driven by the number your odometer is off, in my case its .2, then add what you get back to the number of miles you have driven and that will give you better results calculating your fuel efficiency

Hopefully that's not to confusing I know there has to be a way using the circumference of your tire to calculate it mathematically but idk that's just how I do it

http://go.jeep-xj.info/HowtoSpeedoGears.htm

Here you go, gents

Yep I lost ya on that one haha
Hopefully the new speedo gear will hope. Sucks cause I been running without one for awhile now. I even replaced plugs and everything. I'm thinking of getting a new fuel filter too

thanks but most of that was like over my head.

Why not do this.. fill the truck up. use an app on your phone to calculate actual mileage for distance traveled..drive around a few days even if you get to half a tank. Fill up. Take your gallons and divide them by the distance recorded on your phone..

phones not that fancy but im gonna see if i can borrow a friends gps.

By law a speedo can be off 10% fast or slow from the factory so they are never really right.

Due to tire size the speedo is only off 6.9%.
If your speedo read 220 miles you actually went 235 miles and if 18.2 gallons were used that is 12.9mpg.

31"-29"=2"
2"/29"=6.8966%