At 11:43 AM 10/3/2009, Max Wellhouse wrote...
>Has anyone tested the accuracy by spinning the speedo on the
>work bench with a drill at say 60 rpm and have a value for that in
>MPH?

The speedo RPM for 60MPH is shown at the bottom of the gauge, something
like 805 (it's revolutions per mile, same as 60 MPH).

The problem is how do you know your drill is going 805 RPM?

I've "calibrated" by using a GPS to determine my actual speed. I drove
so the GPS indicated 60 MPH, and noted what the speedo read.

Back home, take it out of the cluster again, and remove the back for
older speedos, remove the impulse sender for newer ones. If you look at
the mechanism, there's an outer fixed metallic shell, then a ring which
moves with the needle, then a center portion which moves with the
cable.

Gently move the needle to the position you noted above. Take a
permanent marker and mark the position on the ring and outer shell. I
then wedged a toothpick in between the ring and shell to hold it in
position. Remove the needle, double check that the marks are lined up,
then put the needle back on, pointing to 61 MPH (so it indicates just
over the actual speed). Check the marks again, and push the needle on
firmly if they're still OK.

Reassemble, and you should be done.

This makes it accurate at the calibrated speed. That doesn't mean it
will be accurate at any other speed. There's both an offset and a
slope. The above calibrates the offset. Slope is calibrated by tweaking
a spring, and/or the magnets in the mechanism, either should be left to
a competent speedometer shop.