The rev counter is going to be muffled to compensate for some volatility in the revs, so you can't rely on it.
Find out what the real revs are with the use of an on-board diagnostic information (OBDII) device. Incorporate the Forscan app or software into your purchase.
Follow the on-board diagnostics, a meter connected to the battery, an on-board diagnostics and information system (OBDII) tool, or a meter located in the car's electrical outlets to complete the test. Take every test.