I know it's reading low because my standalone ECU and the tach on the dyno both say 7300 RPM, when my tach says 8000 RPM. Is there a way to calibrate it?

 

grannysspeedshop.com has a writeup on recalibrating for V6s and V8s but you can use that for calibrating for rotary as well.

IMO don't bother, just get a quality tach like a Sport Comp. The stock tach actually is accurate in 1st gear, unless your car is really quick in which case it actually lags behind the engine, despite being 10% too high under steady state conditions.

 

Thanks, man -- an excellent write-up! Just what I was looking for.