My speedometer is off. I have all the specs, but haven't found the right formula. The tranny is a Turbo 350. the rear end is 336 - the tires are 28" tall with circumference of 87.9 - revs/mile=721. How many teeth should be on the transmission gear?
You have to have a workable ratio between the output shaft and the cable--only changing one may not get you as close as you like to be. Plus, if I recall, the equation requires both gear teeth numbers. It's on the Net, just google for it.
Even then, it'll be impossible to get it dead nuts accurate. The only way to stay legal would be to run a second speedo which measures against GPS location
I had the same issue when I swapped from a three speed manual to a TH350 in my 71 truck. I might have been doing 50 MPH and the speedo said 100 or so. Here is the link that I used to calculate what driven gear I needed (the one on the end of the speedo cable). TCI also sells those gears, imagine that. I got the gear through Summit Racing. https://www.tciauto.com/speedometer-gear-calculator You'll have to know what you described above, plus the number of teeth on the drive gear. Easy enough, pull the speedo gear out, shine a light in, and see what color it is. The color codes to number of teeth chart is on the same page as the calculator. Hope this helps.