Stock '85 Diesel tach accuracy?

[b13bliss]

Since it runs off the W terminal on the alternator, I wonder if there are issues with accuracy.  When I calculate by gear ratios and tire size, and measure my speed with a GPS (speedo is accurate to within 1mph), the calculated rpm for 60mph is about 2550, but the indicated rpm is 2900.  What's the best way to check tach accuracy?

[tylernt]

As far as I know, there are two ways to adjust the tachometer indicated RPM. One is a non-stock pulley and/or belt size, and the other is a pot or two on the back of the tachometer. Check this out:

http://www.vwdieselparts.com/forum/viewtopic.php?t=5263

[b13bliss]

Thanks for the help.  It sounds like there may be an adjustment through a potentiometer existing (or does one need to be added?).  I guess I'll take a look when I pull the cluster to repair the intermittant odometer.