I know that makes a huge difference. I did a road trip last spring where I drove 360mi in 4.5 hours (avg 80mph) and got 11mpg, did the same drive back averaging 65mph and got 15.5mpg, but took 5.5hrs. Now with my mods I can get 18mpg keeping it under 65.
It's a One-Way Street: About the only way to relate engine rpm with fuel economy requires a chart of the engine's Brake Specific Fuel Consumption (BSFC) curves then make adjustments in gearing and speed to keep the engine rpm running at the lowest region of the curve with the widest throttle setting to maintain the operating condition. The BFSC of a throttled engine (all SI gasoline/propane/nat gas/alcohol engines) will be all over the map with the worst of the values coming at high rpm and low manifold pressures and the best values occuring at wide open throttle and middle rpms. Diesels are much kinder with almost all examples coming up best around 1900 rpm. FYI: There is an "instant econ" guage available that plugs into the OBD2 scanner port that will display fuel economy at real-time. ScanTool.net LLC - ScanMaster OBD Software - OBD Software - ScanTool.net
