Cars, RPMs and fuel economy

NIMAN

I am not technically minded when it comes to cars, but could someone explain how it is that one car I drive at 60mph the revs sit at around 1900-2000rpm, yet on another they are at 3000rpm.

I know that one is newer - but why did the older car need the revs to be at 3000rpm at 60mph?

C4Kid

Are they both running on petrol?

One thing I've noticed is, petrols run at higher rpms than diesels, something to do with lower compression ratios,

eg 1.4 petrol @ 100 KMH 3000 RPM tachometer goes to 7500 RPM
1.9 diesel @ 100 KMH 2000 RPM tachometer goes to 5500 RPM

dharn

it has to do with gear ratio if you were driving at 100k in first gear your engine revs would be very high but if you were driving in fifth your revs would be very low

Matt Simis

Have fun:
http://www.bokchoys.com/differential/e46_gear_ratio_calculator.htm

Gear Ratios is indeed the cause. Large engines typically produce more power leading to lower ratio gears leading to less engine load and longer engine life.
Diesels have a narrow power band and a limited total RPM range, they will always be at lower RPM at cruise but also at every other position too. Diesels gearing is setup to keep it in its lower power band, it has to be there to make any sort of progress.

Nothing to do with the age of the car, my 18year old BMW is at 2900RPM at 100mph.

NIMAN

Thanks for the replies, interesting reading.

The higher rev'ing car is a 2001 1.4 petrol
The lower one is a 2010 1.6 diesel.

BostonB

You also need to look at the rpm an engine produces its max bhp and torque. Diesel and Petrol cars are quite different in this regard.