Speedo Gear Help

I have a 1979 Chevy Monte Carlo with a Turbo 350, a 1000 revolution per mile speedometer, 26" (215-70R-15) diameter tires, and 2:73 rear axle ratio. The problem I have is the drive gear that goes into the tailshaft of the tranny is missing. Every website I visit assumes that you have some sort of gear already in there and they can help you figure out how to change it to get the proper reading. I just need to know what gear I need to buy in the first place. I would also like to know what calculations are used to figure this out as I plan to change the rear tire size in the future to something in the 255 60R size range. Thanks.

I used to go to a local tranny shop when I lost a clip or spring or whatever. They would just give me the little parts......maybe your local shop will just give you a gear.....then if you had to order the correct one you have something to go by, and you can tell them how far your speedo is off..