Todd,
Seeing the type of dyno equipment you are testing on makes a suggestion possible.............Dyno tach will be accurate SO,
You can take the tachometer off the car and test it against the dyno instrumentation. That will give you some data that is worthwhile on the track. Even the best electronic tachometers are +/- 5% full scale devices. Do the math on whatever the full scale value is.....can make quite a difference on the track.
Not many folks know that little fact and they assume the high dollar tach they bought is spot on....don't guess, test.
Have a great time testing!
Regards,
HB2
Dissident