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.
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