Sorry guys, but I've got to disagree with you on this one. The tachometer is the gauge indicating the R.P.M., and NOT the total number of revolutions an engine has done
The O.E.D agrees with me
Tachometer
I'd say the the readout in the centre of the gauge is a Rev Counter, because it counts the total number of revolutions that the engine has done.
However, I agree that it seems to be common practice to refer to the gauge that measures engine speed in R.P.M as a rev counter, rather than a tachometer
Brooklands