I have been in an on going debate regarding re gearing and how to calculate milage after the fact.
I just helped a friend of mine regear to 4.88 with a locker in the rear. He has a 92 extended cab 22re 4x4 with a solid axle conversion and 33 10 50 x 15 swampers
The debate goes as follows -
After the regear from the stock 4.10 ratio to 4.88 he assures me the MPG tire size calculation is the same, this calculation - the truck came calibrated for 225/75/15 a 28" tire, his 33 is 32.7". 16% larger, so at 50 on his speedo he is really doing 58, same on his trip odometer. If it says 250 miles, and it took 14 gals, you would think 16.7 mpg, add the 16% mileage and 290 miles is 20.7 mpg.
However, with the regear to 4.88 from the stock 4.10 is his rig is now calibrated correctly?...I think. SO there is no larger tire size correction needed. Is this correct? Do you still need to compensate for the larger tire size when dealing with gas milage or am I totally confused??

This debate has got me questioning this.

 

If you do a little searching you can find sites to calculate speedo error, measuring the tire isn't accurate, most manufactures give an RPM/Speed figure you can use to calculate the real tire size while it's rolling.

 

28" to 32.7" tire would need to go from stock gear of 4.1 to 4.79. So your actually a little lower geared than stock. Your also gonna have to factor in the tire weight and other misc stuff. You should be close though.

 

33 / 28.3 = 16.6% larger tires
4.88 / 4.10 = 19% lower gears

Although usually 33s are actually ~32.5" IIRC, which would make it 14.8% larger tires. So you are geared 2.5% - 5% lower than stock, depending on the actual tire measurement, and would need to subtract that % from your calculated fuel mileage, and speedo reading.

 

You can plug numbers all you want into a calculator but from my experience your buddy needs to get some miles traveled using a GPS, fill the tank and than do the math...
That being said my speedo with 5.29's and 35's is accurate at 50 according to my gps

 

What the guy above said.

Gallons put in/miles he drove = mpg

I don't worry about the speedometer I just go.

:wabbit2: