RPM's at 80 MPH
#331
#333
its not uncommon for the tach to be off by that much though... I dont know about being off to another car - but off from actual rmps (as noted on several dyno pulls)...
#335
#338
^That post kinda sounds like you're saying dire diameter would affect your RPM or make your tach read wrong. I know what you mean though. Changing tire diameter would change the ratio between RPM and actual speed. It shouldn't change the ratio on the dash though since the speedo works off the tire RPM (independant of speed/tire size) but if your were compairing RPMs to speed on a GPS then it would.
#339
#340
^Add to this the fact that you could have a +/- 100 RPM inaccuracy in the tach combined with a +/- 5 MPH inaccuracy in the speedometer... and if they're inaccurate in opposite directions... well, you get the idea.
Really the best way to get an accurate reading is with datalogging software showing your true RPM and a GPS to show your true speed.
Really the best way to get an accurate reading is with datalogging software showing your true RPM and a GPS to show your true speed.