What does RPM mean in a car?
Thursday, 27 June 2019
RPM is many things to many people. A set of numbers to monitor?
To skite about? To ignore? Or another TV show about fast cars?
RPM - for Revolutions Per Minute - were once a larger part of the motoring game, quoted from start to finish, analysed and adjusted, compared, calculated and hot-rodded.
For RPM is about the speed of an internal combustion engine, how many revolutions a minute it’s running at, how many times it fires, and pistons and shafts and belts and such spin around in 60 seconds; it’s usually shown on a tachometer sitting alongside a car’s speedometer.
In the right gear ratio, high RPMs bring more power and road speed.
Engine torque - the pulling power for getting away from the lights or lugging over big rocks - is found a bit lower in the rev range.
An engine’s speed is governed by the amount of throttle applied, road speed is then governed by those RPM and a car’s gearing.
High revs in a low gear is for acceleration, high revs in a high gear is for road speed.
Spirited drivers once had to watch the redline - where the numbers went from black to red - on the tacho.
Exceeding maximum RPMs could blow an over-wrought engine while most modern cars, with computer-controlled engines and transmissions, have an automatic, often jarring, rev-limiter if a driver pushes too hard.
What’s the average RPM?
A modern passenger car cruising down the highway, in top gear, could be running somewhere between 1500rpm and 2000rpm at 100km/h.
And a diesel ute idles around 750rpm while in sixth gear sits around 2000rpm for 110km/h.
Most of today’s road-going petrol engines can rev out to around 6000rpm.
While that’s high for many diesels the rev limiter, or redline, can be easy enough to hit with a petrol motor when accelerating through the gears.
Formula One V6 engines may run out to 15,000rpm for close to 1000 horsepower and top speeds around 350km/h.
Using RPM for fuel savings
In today’s motoring world much of the driver’s work is controlled by computers which try for the best fuel and emission efficiencies.
Yet with a little awareness a driver can still help out, finding sweet spots on the tachometer and speedometer for a balance between performance and fuel economy.
The trick is to change up a gear as early as possible - while maintaining road speed - and to change down when an engine is labouring.
This is easiest with a manual gearbox but not impossible with automatics with a ‘sports’ mode and, once those sweet spots are found, easy to watch for on the tachometer.
Talk to an AutoGuru expert for an idea of the best highway RPM for your car.
Bruce McMahon is a Queensland-based journalist who’s spent a fair slice of his career dealing with automotive matters.
His first car was a 1949 Riley Roadster, followed by a mix of machinery from Porsches to Jeeps, Alfa Romeos and Range Rovers through to the current four-wheel drive Mazda ute.
He’s driven the Nurburgring and the Tanami Tracks.