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  #26  
Old 03-17-2005
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Quote:
Originally Posted by rwenzing
If it's in the axle housing, use the formula I gave for speedo correction:

1-(28/33)= 0.15 = 15%

So, your odometer is reading 15% too low with the 33s in palce of 28s and your speedo error should be about 10mph low at 70MPH.

The gear ratio change at the diff doesn't matter because the axle measurement is reading actual wheel RPM. The only thing that changes that is tire size.

[On the 2001+ Rangers, changing the axle ratio does affect the speedometer accuracy because the measurement is taken at the trans output shaft, not the ring gear.]
Hmmmm....

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?

Thanks
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Old 03-17-2005
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Quote:
Originally Posted by solodos
Hmmmm....

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?

Thanks
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
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