Speedometer Cable Driven Gear
 
When you change the rear end gear ratios or tires from stock, your speedometer may and probably will read faster or slower than your actual vehicle speed. Whether the transmission is an automatic or a standard, the basic setup is the same. The speedometer cable has a small plastic gear, called the driven gear, that attaches to the end of the cable with a metal C clip. The speedometer cable inserts into the transmission tailshaft housing and meshes with the tailshaft gear to spin the speedometer cable and make the speedometer indicate your speed. For 1970 Mustangs, the driven gears are different in the angle of the teeth between automatic and standard transmissions and where they insert into the tailshaft housings. The chart above shows the differences. The end of the speedometer cable is held in place at the transmission by a small bolt and clamp. To change how your speedometer reads, you change the speedometer cable gear. When you look at the driven gear, you will see a number on the end. This tells you how many teeth this gear has.
The basic rules are these:

1. One tooth on the driven gear approximately equals 5 MPH.
2. If your speedometer reads too fast, add teeth to slow it down.
3. If your speedometer reads too slow, subtract teeth to speed it up.

Absolute accuracy can vary but this is the basic technique. A much more detailed explanation of this issue can be found here.