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