Right, All of those things are important too, but RPM is a way of making sense of all that and giving you a good guess as to where economy can be found.

My point is that if the engine has enough HP and torque to move the vehicle at the seed you want at the ideal RPM, (assuming you chose the right engine to do the job) then that is about where you will find the best mileage you can hope for for that speed and weight of vehicle.

Compare driving a suitably light vehicle at 65 mph and 1,700 rpm
To driving the same one at the same speed, but at 3,200 rpm.

Assuming the engine is comfortable at the low RPM, and not lugging, then clearly you will get better mileage that way.

So, yes there are a bunch of factors, and ideal operating RPM is one of them.

This Cummins info Secrets of Better Fuel Economy; The Physics of MPG has lots and lots of tips and suggestions and factors on making good mileage in a truck, big trucks, but most all of it applies to any vehicle: http://www.everytime.cummins.com/assets/pdf/MPG_Secrets_Whitepaper.pdf

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-