Peter Bennett wrote in
news.com:

Talk to a mechanic to see if he has some independent means of
measuring engine RPM. I recall seeing a portable mechanical tach that
could be held against the end of the crankshaft.



Strobotac. Mine's circa 1960. Turn the dial until the flashing xenon lamp
makes the shaft look like it's standing still, then simply read the dial.

Most tachs are way off...especially if the belt slips a little.
The same guy who calibrated the fuel guages calibrated the tachs, I think.

--
Larry

You know you've had a rough night when you wake up and your outlined in
chalk.