Speedometer Ratios

Finally, let's look at speedometer ratios.

Suppose we put an exchange transmission in a car, and now the speedometer is off, because the speedometer drive gear has a different tooth count. What do we have to do to the driven gear to correct it?

Let's say the old drive gear had 7 teeth and the old driven gear had 21 teeth. The exchange unit had 8 teeth on the drive gear.

Old Drive Gear divided by the New Drive Gear = Ratio

7 teeth divided by 8 teeth = 0.875


Old Driven Gear divided by Ratio = New Driven Gear

21 teeth divided by 0.875 = 24 teeth


A 24 tooth driven gear will correct the speedometer error.


Let's do one more speedometer change. This time the old drive is 9, and the new drive is 10. The old driven gear is still 21.


Old Drive Gear divided by New Drive Gear = Ratio

9 tooth divided by 10 tooth = 0.9


Old Driven Gear divided by Ratio = New Driven Gear

21 tooth divided by 0.9 = 23.33 teeth

We can't get a 23.3 tooth count so we round it off to 23 teeth. Now the speedometer will be close, but not exact, because we had to round off the number.