Is the RPM on the tachometer measuring the revolution per minute, RPM, of the transmission or the engine. If transmission than is it measuring whichever gear you're in or is it measuring the fly wheel/inputshaft. Does it make a difference between auto/manual? If engine than is it measuring the crankshaft's RPM?

 

The tachometer measures engine speed. There is no difference as regards transmission type or shifter position.

 

When I had my car smogged recently, the tech hooked up a timing sensor over the valve cover and went to work. I noticed the numbers were lower on the computer than the dash tach. The computer registered 3,000 RPM and my inside tach said 3,200 or so. I wonder how accurate my speedometer is, BTW.

 

hmm...so it probably is measuring the crankshaft or the inputshaft to the transmission then?

wow and I've thought that only the speedo is off for safety reason. Wonder what's the reason behind the tach being off.

 

The tach meaures the firing pulses of one of the cylinders to calculate engine speed.

Why would the speedo be off for safety reasons? Wouldn't it be safest to know exactly how fast you're going or exactly how fast the engine is spinning?

Anyway, the inaccuracies in these instruments are just a result of their design. To make them extremely accurate would make them more expensive, that's all. They work well enough for most peoples' needs.

 

cool thanks...so it has nothing to do with measuring gears/shafts spinning.

I would think that at high speed the speedo being off on the high side is a safe way to make people think they're going faster then they really are so to discourage them from going faster.