Speedo / Tach / Gauges: Pinion Gear

[mjeansonne]

I have new 205 75 R 15 tires on the 62 Hawk. The speedometer indicates 5 miles per hour faster than I am actually going, gaging with my GPS. To correct that inaccuracy, do I want to increase or decrease the number of pinion teeth and if so, by how many teeth to make up the 5 mph discrepancy?

Thanks

[52 Ragtop]

Look in your chassis parts book, there "should" be a chart for the correct pinion gear for the rear axle that you have. That should give you an idea what it "should" use. start out by one or two teeth. To get it "perfect" you'll end up with a couple of spares!

Jim

[JohnM15]

This could easily be calculated if you know the following:

1. Both current speedometer gears (pinon and drive) or ratio of the drive shaft to speedometer
2. Axle ratio
3. tire radius

Personally, I would use MS Excel to do the calculation as it could show which available gears would be closest... Let me know if I can be of assistance with Excel.

[swvalcon]

The nice thing is it may keep you from getting a speeding ticket.

[WCP]

Start by adding one tooth to the pinion to slow it's rate of rotation. The rate of rotation can be calculated by dividing "x" by the pinion teeth, where "x" is the number of transmission gear drive teeth. This will be either 7 or 8 depending upon your rear axle ratio and type of transmission and can be found in the parts manual. By determining the rate of rotation for two different pinion tooth counts, you can determine the percentage effect on your speedometer.