Assuming it's a naturally aspirated petrol engine, best efficiency will occur between 2000 and 3000 RPM and 70-80% of available torque at that speed.

You'll find that peak torque occurs at a higher RPM usually 4000-5000RPM. This is where best volumetric efficiency occurs. But best engine efficiency is usually below this.

Load is much more important than engine speed, so don't totter along at 30 in 3rd gear because that gives you the 'magic' 2000RPM. 1500 RPM will give better efficiency.

Turbo/supercharged engines can vary considerably. Probably around 2000-3000 RPM but 50-70% of available torque.

For an automotive turbo diesel best efficiency is always between 1500 and 2000 RPM and about 90% load.

Edited by unthrottled on 22/03/2012 at 14:17