MPG discussion with 33s
CF Veteran
Joined: Jan 2016
Posts: 1,242
Likes: 41
From: Newport News, VA
Year: 96 & 88 4 dr Cherokees
If you've changed tire size and not the speedo gear or diff gears, calcuate your mpg as normal (miles driven on speedo / gals used)....let's say that gives you 10 mpg
Now, take the current tire size and divide it by the old tire size...lets say 31" / 29", or which gives you 1.07.
Multiply the speedo miles by that number to get actual miles traveled due to the bigger tire, then figure out your mpg again.
In this case, you'd be @ 10.7 mpg
If you changed diff gears, but not speedo or tires, do the same thing basically, but divide old gear ratio by the new gear ratio (3.55 / 4.56 = .78), which is how far off your speedo is for speed (55 mph speedo x .78 equals actual speed, or 42.9 mph)
.
Now, take the current tire size and divide it by the old tire size...lets say 31" / 29", or which gives you 1.07.
Multiply the speedo miles by that number to get actual miles traveled due to the bigger tire, then figure out your mpg again.
In this case, you'd be @ 10.7 mpg
If you changed diff gears, but not speedo or tires, do the same thing basically, but divide old gear ratio by the new gear ratio (3.55 / 4.56 = .78), which is how far off your speedo is for speed (55 mph speedo x .78 equals actual speed, or 42.9 mph)
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Last edited by TRCM; Mar 17, 2017 at 11:46 PM.
Thread Starter
Junior Member
Joined: Dec 2015
Posts: 44
Likes: 0
From: Indiana
Year: 2000
Model: Cherokee
Engine: 4.0L I6
In order to get an accurate reading, you need to change your speedo gear to your setup. By adding bigger tires you have changed your final drive ratio. You can use one of the charts out there, such as this one, http://www.grimmjeeper.com/gears.html to figure out what speedo gear you need for your tire selection, trans, and stock gearing, but the only way to make it as close to stock as possible, is to change gears in your diffs to match your tire size, AND change your speedo gear as well.
Your numbers are wrong.
Your numbers are wrong.
