Every tach is different, but I've had good luck using just the battery charger on the bench. My S10's tach is dead-nuts on, I verified it with the ALDL datalogger. I'm usually too worried about trying to keep the nose/rear of the truck straight to look at the tach though @ WOT.
It's also worth noting that I don't generally strive for NASA / Aerospace quality accuracy
If you can't tell from most of my posts, I'm pretty shadetree.
V6'er, what's the best way to generate multiple frequences on the bench to test? I wonder if something like the Megasquirt Stimulator could be used (since it generates RPM signals to test your Megasquirt with)?
I'm using the Avalon module, and did the turn car to ON -> place the needle on. It's off by about 200rpm no matter what I do (200 too high, verified with OBDII logger). I'm thinking about recalibrating the MR2 module, because it's kind of annoying.