I found this on the novaresource.org site so it may be of some help at least.
OK. Now you know who to fix the problem but how do you know what size gears to buy? Simple. Patrick Coxe has solved that problem by developing a formula and a table to determine the correct speedometer gears needed for a given combination of axle ratio and tire diameter. According to Coxe, the formula for determining the proper gears for any application consists of multiplying the rear axle by 20.2, and dividing that number by the tire diameter. (rear axle x 20.2 / tire diameter) Then, simply look up a ratio number on the on the following table (large and small gear versions) which is closest to the calculated number. By installing the gears, you should be close to the correct calibration.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars
I found this on the novaresource.org site so it may be of some help at least.
OK. Now you know who to fix the problem but how do you know what size gears to buy? Simple. Patrick Coxe has solved that problem by developing a formula and a table to determine the correct speedometer gears needed for a given combination of axle ratio and tire diameter. According to Coxe, the formula for determining the proper gears for any application consists of multiplying the rear axle by 20.2, and dividing that number by the tire diameter. (rear axle x 20.2 / tire diameter) Then, simply look up a ratio number on the on the following table (large and small gear versions) which is closest to the calculated number. By installing the gears, you should be close to the correct calibration.
