Because of unknown rear end ratios, tire growth at speed, compounded errors
in measuring, or wide allowable tolerances in tire manufactures data, the most
direct way of determining the proper number of teeth in the speedometer cable
is to actually test it. Assemble the car with the gear you now have and drive
it on a highway that has mile markers. Drive at about 60 MPH for 10 miles and
record the mileage as close as possible. Then use the following formula to
determine the number of teeth needed on the driven gear.
Teeth on your present driven gear X Miles recorded/10 = number of teeth
needed on new driven gear
as an example: If you have a 16 tooth driven gear and you record 12 miles
during the drive the calculation is;
16 X 12/10 = 19.2
You would therefore use a 19 tooth gear or what ever is close.
After you check out the new gear and the odometer is measuring the miles
correctly, drive at 60 MPH and you should pass a mile marker once a minute. If
there is a speed error than the speedometer needs to be adjusted. If it's not
too far off you may be able to adjust the needle return spring.
As you have been discussing, there are many formulas that can be used to
calculate the ratio but they are all subject to the accuracy the input data.
John Logan
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