Lesson 5 of 12 · Automotive basics

RPM: count the revolutions

3 min20 XP3 questions

A tachometer counts engine rotation. What can that reading tell you on its own?

RPM means revolutions per minute. For an engine, it usually describes crankshaft speed. Engine speed and road speed are different: the selected gear, final drive and tire size connect the two.

The idea

RPM counts revolutions in one minute. Road speed also depends on the gear, final drive and tire size.

A four-stroke cycle uses two crankshaft revolutions. RPM is not a direct reading of vehicle speed or power.

Think it through

The crankshaft holds 2,000 RPM for 30 seconds in a thought experiment. How many revolutions does it complete? Can this alone give the road speed?

Show the reasoning

Thirty seconds is half a minute: 2,000 × 0.5 = 1,000 revolutions. Road speed remains unknown because the drivetrain ratios and tire size are not supplied.

Take away
  • Convert the time to minutes before counting revolutions.
  • Do not read RPM as road speed or power.
Sources