When I was test driving the truck for a couple weeks before I bought it and before he put the gauges in I was seeing 240-260 miles per tank of 20-22 gallons. That was in the summer and it is now winter, but dropping damn near 100 miles per tank just because of the cold is outragous. Is there anything else besides tire size that could cause the odo to read low?


Mike

 

There is a segmented reluctor ring on the output shaft of the transmission or transfer case. This ring has 40 teeth on it. A magnetized sensor detects changes in the field each time a tooth travels past. The PCM contains a value of how many pulses equal one mile. Based on this fixed value, the PCM can translate pulse rate into speed.

To calculate, divide the number of inches in a mile (63360) by the circumference of your tire...

( circumference is pi*d, so for a 31" tire for example, it is 3.14159 * 31 = 97.52" )

So, 63360 / 97.52 = 649.71 = The number of times the tire rotates in one mile.
Then multiply that by the gear ratio, for example 3.73:

649.71 * 3.73 = 2423.42 = Driveshaft revolutions per mile.

Then multiply that by the number of teeth on the reluctor ring (40)

2423.42 * 40 = 96936.8 pulses per mile for a truck with 31" tires and 3.73 gears.

If this number is wrong or if the gear ratio is programmed wrong in the PCM the odometer and speedometer will be incorrect.

It is also possible to falsify the gear ratio and factor the pulses per mile constant to support different sized front and rear tires while keeping ABS happy and the Speed/Odo accurate. I can help with that if necessary.