Engine size + RPM + MPH = Best MPG?

[SteveSubaru]

Has any study been done on calculating the most efficient speed based on RPMs for the best MPG?

Obviously, terrain & wind (head/tail from Mother nature, not wind resistance) affect MPG which the driver has little control over. Additionally, the biggest factor in getting the best MPG out of ANY GIVEN car is the driving style (no jack rabbit starts, anticipate stop lights, no hard breaking,....).

The question is "At what combination of RPM and speed with a given engine size (4, 6, or 8 cylinders) give a driver the best MPG?

Assumes: all other factors are the same (wind, hills, driving style).

It is my understanding (?) the optimal speed for the best MPG is around 45-50 mph. What I am looking for is a more definitive answer relating speed and RPMs to best MPG.

Example for clarification:

Obviously, a driver going 20 mph at 2000 rpm is not getting as good a mileage as one going 50 mph at 2000 rpm. (No brainer) This is the difference between driving in say 2nd gear and 4th/5th gear.

But a driver going 70 mph at 3000 rpm is covering more miles for the additional rpms (gas consumption) over the 50 mph/2000rpm driver. Were the additional 1000 rpms worth the additional miles covered in terms of MPG?

If a driver could go faster and not increase the RPMs significantly, the car could achieve a better MPG. Where is the break point in RPMs for a given engine size (4,6,8 cylinder)?

Any charts or graphs out there which show this information?