Since it seems this business of getting correct mechanical speedometer readings is highly obscure, here are some more details for the record.
This '87 Chevy Suburban has the 454 engine and TH400 (turbo hydromatic) transmission. My mechanic says that for this transmission there are only 3 speedo drive gears available, with 38, 37, or 36 teeth.
A difference of 2 teeth allows for a correction of at most
38/36 or 5.6%. My speedo is off about 30%; to correct this large an error a transducer may be installed on the cable from transmission to speedo. As it happens, there already is a transducer installed; obviously it does not have the necessary ratio.
When actual speed is 65 mph, indicated speed is 50 mph, so
50/65 = 0.769. The goal is to increase this ratio to 1.0. This ratio is occuring with 38 teeth and transducer ratio
0.833. Changing from 38 to 36 teeth helps slightly: 38/36 = 1.056. So with 36 instead of 38 teeth, the error ratio will be 0.769*1.056 = 0.812. It is very close to the 0.833 ratio of the transducer now on the cable. If I remove the transducer I'll get an error ratio of 0.812/0.833 = 0.975, which is just 2.5% slow. A transducer with ratio 1.026 would be exactly right, but if none is available, having no transducer at all may be good enough.
How accurate does the speedo need to be?
Una