Check the link for more info.
http://www.geocities.com/mgnedell/notchback/tech/speedo.html If your car has the stock drive gear inside the tranny, it says you need the 23 tooth. However, those don't seem to last for long. Better choice would be to pull the tranny tail case, and replace the stock 8 tooth drive gear on the output shaft with a 6 or 7 tooth. Doing so would let you use an 18 tooth (with the 6) or a 21 (with the 7). I've done this twice on mine - our undercarriage is different - I couldn't do mine with the tranny in the car; you may be able to. If you don't want to go through all that, contact one of the many speedo shops across the country that make speedo converters. It's a little black box that plugs into your tranny, and your speedo cable plugs into it. They'll tell you how to measure the % that the speedo is off, and they'll gear the box to correct it. If you have to pay someone else to do the speedo work, the black box will be cheaper. If you do the tranny work yourself, replacing the gears will be cheaper.
