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Try this formula:
MPG=1204*MPH/(L*RPM*VE)
L: Liters of engine displacement
VE: volumetric efficiency of the engine
Usually the volumetric efficiency of the engine at low constant vehicle speeds is about 0.30 but when I read about the Oldsmobile Turnpike Cruiser I discovered that the VE was only 0.26 when cruising slowly. High compression ratios help to reduce the fuel consumption of engines running at light loads. When the throttle is completely open, the VE of an engine without a turbocharger or supercharger should be close to 1.0 when the engine speed is near the torque peak. Using a VE of 1 in the formula will let you estimate the fuel economy when using full throttle.
Lets try an example: A 1999 Saturn SW2. I'll use these numbers:
MPH=25
RPM=850
L=1.9 liters
VE=0.30
Now here is the calculation:
MPG=1204*25/(1.9*850*0.30)
MPG=30100/484.5
MPG=62.126
That seems to match the measured result closely. There are a lot of factors that could make the VE estimate of 0.30 inaccurate. For example: unusually high gears, unusually small engine, unusual compression ratio, and installation of a high performance camshaft.
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