Here is how to bench calibrate your speedometer after you do
some service. You need simple tools and a speedo or tach cable
and a drill.
First take the guts of the speedo out of the case.
Next, You must know the engine RPMs at a certain
speed in 4th gear (no OD). In my car that is about 3350rpm for 60mph.
because I have a 4.1 differential. Others will be around 3000/60. Divide
this by 2.5 to calculate the speedo cable RPMs (here it is 1340). So 1340
cable RPM equals 60 MPH. Now hook up a cable to a tach and a drill
to the other end. Run the drill at max RPM, and look at the tach reading.
It ought to be fairly stable. The drill is really turning 1/2 the speed
indicated on the
tach. Calculate what MPH that RPM should be. In my case my drill was turning
1200 rpm: the speedometer should read (1200/1340)*60 = 53.7 at that cable RPM.
Now, to adjust your speedo turn the pointer to the speed you saw when running
the drill. Then hold the disc in the guts that turns when you move the needle.
Hold it gently. Now push the needle to the speed you calculated it OUGHT to
read
while holding the disc at the original position. Now run the drill again
and fine tune the needle position. When it reads the correct speed when running
the drill you are done. Reassemble the guts into the case Clean off dust
and put the glass back on. Reinstall the speedo and test drive. It ought to
be
REALLY CLOSE if you did it right, and if your drill runs at the same speed
always.
Hope this helps.
-Tony
ARhodes@compuserve.com
Adjust the speedo to read 53.7 when you are running on the drill at max rpm!!!
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