instrument repair and supply houses, and are custom-built, taking into
account the vehicle’s final-drive axle ratio and tire size. An easier and
less expensive way to accomplish the same thing would be to check your
speedometer while driving against a vehicle with an accurate
speedometer, making note of the difference in readings, which will be
consistent at any speed. You would then know that your own
speedometer reads 5 mph slower than actual road speed, which might
save you from getting a speeding ticket in a school zone.
As mentioned, the speedometer itself is often called the “head.”
Although we now know that a lot of what makes a speedometer work is
down in the transmission and drive cable, a lot goes on in the head, as
well.
Figure 6
Most of the time, and for practical purposes, home repair of the
speedometer head will be limited to simple cleaning and lubrication.
A word of caution: While most speedometer heads can be successfully
opened, cleaned and lubed at home, you should never open an M-series
speedometer with radium glow-in-the-dark face markings, because of the
danger of radium (radioactive) dust. This is sometimes confusing,
because many World War II and some M-series speedometers have
luminous paint numerals and needles instead of radium markings. An
easy way to tell the difference is that a speedometer with radium
markings will glow in the dark under all conditions, while a
speedometer with luminous paint markings will only glow brightly after
being exposed to light, and after awhile this glow will fade. If in doubt,
do not open the speedometer.
If the speedometer on your vehicle either just stopped working or has
never worked at all, chances are good that the fault lies in the drive
cable and is generally a broken inner core (Figure 6). This is usually easy