Ok, so I was just reading a Motor Trend article on a '91 LO3/M5 RS, and they claimed 60 MPH read 1,600 RPM on the tach. My car is in the body shop right now, so I can't check it...but I know my tach reads just shy of 2,500 RPMs at 75...and yes, I believe the gears are original, as the car was never modded before me. Is my tach accurate, or am I off a little?

Also, while I ALWAYS turn on my A/C before it gets this hot, it appears my engine fan DOES turn on without turning on the A/C, but it does it well over about the 240 mark on the gague...as far as I know, with a new fan switch my uncle installed that appears operable, can I assume my gague is off, or is it possible even my new OEM switch is screwed up and its turning on too high? I always turn the A/C on when it reaches 225 on the gague in traffic, so that it never goes too high....can I have a shop plug into the ECM while the car is running and compare actual ECM engine temp to the gague to compare?

Thanks to anyone who can help!

 

TTT...anyone?

 

neither are all that accurate.
You can calculate the RPMs based on tire size, gear and rear end ratio to know exactly what RPM should be for a given MPH. There's a calculator in the tech section.

The ECM temp sensor is located in a different part of the engine so it probably wouldn't read the same anyway.