Mods please sticky.

When you change your tires to larger ones for off roading or whatever you need to compensate for this. Assuming you are running stock gears and don't have the coveted 4.77's. You can change the driven gear on the speedo cable at the transfer case. This is the best method. Or just make note of how far off the speedo is by using a gps or another vehicle beside you at 60mph indicated.

I'll help. Assuming a stock 1st gen Trooper with 4:56 gears and 235 75 r15 tires is accurate. Then when you swicth to BFG TA at 31x10.50x15 tires you will be traveking at about 66 mph for and indicated 60. Same tire in 32x11.50 will be about 67mph.

So you can do the math and figure the difference in diameters as a %. But that is often off because the measurement that matters is "static radius" This is the distance from the ground to the center of the wheel. That accounts for the deflection (flat area) caused the weight of the vehicle compressing the tire. It's usually a good bit less than 1/2 the diameter.

For figuring MPG and total mileage I use this backyard method. it is usually more accurate than the % calculations many use based on supposed diameter.

Say you change to 31x10.50's. Use a gps or compare to a knowm accurate car at 60 mph. If you are actually going 66 when 60 is indicated note that.

I then take indicated miles travels since last fill up. Say 277 miles. I then divide this by 60. This = 4.61. Now do 4.61 x The missing 6 mph. As in for every 60 miles you show on the ODO you are missing 6 miles because you were really traveling at 66mph. So in one hour of driving at 60 you really went 66 miles or 6 more than the odo shows.

So 4.61x6=27.69. You are missing 27.69 miles on your odo for that last tank full. So add 277+27.69=304.6 miles actually traveled.

Ok so you filled up 17 gallons. 304.6 miles divided by 17gal=17.92 mpg!

If you had used the indicated 277 divided by 17 gallons=16.29mpg. Lot's of folks do this and that's why they say their mileage went through the floor when they went to bigger tires.

Kinda hillbilly math compared to the schoolboy increase of XX% tire diameter thing. But just as or more accurate because it's real world measurement vs paper tire size.

Using the same formula. 3,000 indicated miles since last oil change is actually 3,000 Divided by 60=50. 50 x 6 missing miles =300 miles. So you actually have 3,300 miles since last oil change. This adds up after a while.

100,000 miles indicated is actually 109,999 miles travels. Those 31's with stock speedo gear are off by 10,000 miles over 100,000!