My days at the Cadillac dealership with Turbo 400 we found that a one tooth difference equalled about 2.5 mph difference around 50 mph. Fewer teeth spins the speedo faster and more teeth slows it down. Figure out how far you are off and go from there. It may not be perfect at all speeds. The speedometer sleeve that the gear goes in will be marked with 4 different numbers for the 4 gears that will fit that sleeve. Ex. 35,36,37,38 will take any of those gears. As you get to larger gears they had to offset the center of the sleeve to compensate for that. Take a sleeve and look at the center point and you'll see what I mean. I believe they may have had different output shaft gears, but I can't remember. I recently still bought some gears. Remember to replace the rubber oring on the outer part of the sleeve and get a new seal for the shaft part of the gear. It is held in by a small clip that you can remove with a pocket screwdriver. The new seal willinstall nicely with a 3/8 socket and put the clip back in on top. If the shaft on the gear is worn you can double stack another seal on top of that one. Sorry for the long message, but thought I would give you everthing I can.
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