This discussion pops up every so often on r.a.d., and once again, I feel the need to chime in... I don't understand why a modern vehicle should behave like yours does, but what do I know. FWIW, I have never observed similar behavior in any vehicle I've driven for extended periods.
I suspect it is far from a scientific fact. At best, it's a very rough correlation. Even then, rough correlations can be useful, but I think this one has little relevance to maximizing cruising fuel economy in a street car or truck.
Did your research discuss part-throttle fuel efficiency or only wide-open throttle (WOT) fuel efficiency? Street vehicles don't cruise at WOT, so without access to a map of BSFC plotted against engine load *and* rpm, the only way to plot fuel consumption vs speed is by accurate measurement.
FWIW, cruising around for a while at roughly a certain speed and then measuring fuel usage some time later is *not* an accurate measurement technique. Without *instrumentation*, either built-in or added-on, any attempt at correlating mpg to speed is (at best) a guess.
If I use the 40% "rule", my car (4th-gen Corvette 6-speed) should deliver best fuel economy at about *100 mph*. It does not. In fact, it delivers best mpg at about 40 mph, and mpg decreases continuously with higher speeds. At 100 mph-40% redline, mpg is roughly half of the maximum.
No way. Increasing rpm (at a given speed) is an excellent way to *reduce* fuel economy. The engine output does not increase (constant speed), but the internal friction power required by the engine increases. IOW, the engine requires more power consumption to do the same work.
Furthermore, increasing cruising rpm at constant road speed will force a reduction in throttle angle (more rpm, same power output), which increases pumping losses. All in all, running higher rpm than required to maintain cruising speed is *bad* for fuel economy.
FWIW, if I drop from 6th to 5th in my car at 65-75 mph, mpg (observed on the mpg readout) will drop by ~20%, even though 5th gear puts the engine at the magic 40% at 67 mph.
I have no way of knowing whether your vehicle does what you think it does, but I *know* that not all vehicles behave similarly. -- Chuck Tomlinson
