From what I understand, pinion gear tooth counts can be calculated as: (5,280 * 12 * DriveGearTeeth * DifferentialRatio) / (pi * RevolutionsPerMile * TireDiameter)

With the ZJ having 13 teeth in the drive gear, a 3.55 differential ratio for my I6, and our speedometers assuming 1,000 revolutions per mile, that means pinion tooth count should be ~931/TireDiameter

This gives 26 teeth for 36" tires up to 34 teeth for 27" tires. Most charts I see indicate 1-2 teeth more than my calculations give though, meaning their recommendations would indicate a slower than actual speed.

So two questions:

1. Am I missing something in my calculation? Only thing I can think of is tire deformation or accounting for future tire wear maybe... Still, I would think the desire would be to err on the side of indicating a higher than actual speed.
2. Is the speed sensor used by anything besides the speedometer/odometer/fuel economy indicator? Anything regarding engine or transmission performance? If so, and if my numbers happen to be correct, would they benefit from having a more accurate reading from the speed sensor?

 

I don't think you are missing anything by your calculations. There is an exactness to the math in these equations. The math makes it possible to derive an appropriate tone wheel tooth count in order to sense the correct speed.

However, there are other factors which influence the decisions being made for the tone wheels actual resulting tooth count. Greater numbers of teeth will produce a higher frequency output signal. This allows a better resolution and higher signal update rate. The number of teeth is chosen as a trade-off between low-speed sensing accuracy and high-speed sensing accuracy, and cost..