Speedo gear changes: There is a formula to correct the speedo that uses gear teeth X mph which I never was able to make work correctly. I have a 1982 Buick Riviera FSM that actually has lots of pages that show trans, rear, tires, and what speedo gears belong together to get it correct. And even then I find I have to make a lot of speedo gear swaps until I get it right. I find the manual trans like Saginaw the easiest, since the inside output shaft gear never needs changing, just the speedo cable gear housing gears. I have a large selection of gears that cover a huge range. I also have the factory reduction gear adapter that they used for crazy rear gear ratios that fell out of the normal range of speedo gears. Automatics are a bit tougher because the gear housing is for a range of gears. You might need a different housing to use the right gear that you require. And even after all that, you take the vehicle up on the highway, have a buddy stay ahead of you with his modern car, and drive a steady 60 mph with the flashers on, check your speedo against that, and I bet you still won't be right on. But that's part of the fun.