Originally Posted by solodos
I see what your saying, but it still seems odd that the gears wotn affect it.
So when I do my fuel mileage and it says I get 15MPG I am really getting 17.25MPG?
The tire is locked to the wheel, then to the axle, then to the ring gear, then to the pulse wheel for the sensor. Whatever RPM the wheel turns must be the RPM that the pulse wheel turns. That's why the diff gears do not affect the speedometer/odometer on your 99.
Later trucks are different because the sensor is at the trans which is on the other side of the diff gears. On those, both the tire diameter and the gears matter.
For a change of 28" to 33", the odometer is reading 15% fewer miles than it should. That is the same as saying that it should read about 17.6% more than it does.
1-0.15 = 0.85
1/0.85 = 1.176 = 117.6%
So, multiply your indicated MPG by 1.176
1.176 x 15 = 17.64 actual