Speedo and rev counter issues

You found the other kink in these Mondials.
The speedo says 10 mph and the tach says 500 rpm at rest on my car. I thought it was just mine, but its on all of these cars. Take the good with the bad lol.
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When I first got my Mondial, my tach was pretty inaccurate, consistently reading far lower revs than the engine was really turning. I removed the gauge from the dash and cleaned it, and re-installed it. It was much more active after that. I paid particular attention to the coiled spring at the back. If you do remove the gauge, be careful with that spring, it's very delicate.

Some time much later (a couple of years later), my speedo began to fluctuate wildly after about 10 or 15 minutes of driving, then stop working completely. The culprit was a dying speedometer sensor. A good clue that it is the speedo sensor rather than the gauge itself is that the odometer stopped working at the same time as the speedometer.

http://www.ferrarichat.com/forum/showthread.php?t=328303&highlight=speedometer+

If your speedo is just consistently reading lower than actual road speed, that does not sound like a bad sensor sending unit. That sounds to me more like the speedometer needing a rebuild.