Okay just thinking out loud here, but your average driver measures what we call mileage or fuel economy in distance driven per volume of gas consumed (not per unit of time).
Just using some fake numbers to throw this question out there: if I use 3 gal per hour in gear X at 30 mph and 4 in gear Y at 60 mph, and travel a distance of 60 miles, then X uses 6 gal in 60 miles (10 mpg) and Y 4 gal in 60 miles (15 mpg). So while the engine may be more efficient in gear X, in the real world it burns more gas over the same distance.
I know it's over-simplified, but I used it to illustrate that maybe if we are talking about the kind of fuel economy that most drivers care about -- that which costs them the least for their total miles driven -- then measuring in volume of gas consumed per unit time isn't telling us anything.
It's Sunday, and I usually turn off my brain on Sundays, so please correct me where I am wrong.
-Al-
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