All RPM are not created equal

The riddle from the previous post is solved. My college roommate responded via e-mail. The remainder of the post is his words:

I plotted quite similar data points for the Omni about fifteen years ago. (In fact, I thought you were the data recorder as I called out the numbers. This was on I-80 on the way to Iowa City.) I tried to eliminate all of the obvious sources of error: I did it all in the same gear--though it was an automatic, I took my first data point after the shift to the highest gear. I did my best to maintain an even speed. I attempted to ensure that I was in "clean" air. And yet, I still got the sort of curve you describe.

I have since accepted--I suspected it at the time, but I liked the curve so much I didn't want to accept reality--that there was a flaw in my fundamental assumption: That rpm are a good indicator of fuel consumption. While there is obviously a close relationship, it is apparently sufficiently inaccurate to completely invalidate experiments like ours. The issue is that the gas pedal was down farther for the higher speeds, and thus the throttle valve was wider open. The engine was enjoying more fuel vapor per revolution. From Howstuffworks:

When you step on the gas pedal, the throttle valve opens up more, letting in more air. The engine control unit (ECU, the computer that controls all of the electronic components on your engine) "sees" the throttle valve open and increases the fuel rate in anticipation of more air entering the engine. It is important to increase the fuel rate as soon as the throttle valve opens; otherwise, when the gas pedal is first pressed, there may be a hesitation as some air reaches the cylinders without enough fuel in it. Sensors monitor the mass of air entering the engine, as well as the amount of oxygen in the exhaust. The ECU uses this information to fine-tune the fuel delivery so that the air-to-fuel ratio is just right.

Apparently the increase in the amount of fuel supplied per revolution more than offsets the savings in rpm. To actually test that you'd have to have some way of reading the data from your ECU. I suspect that is precisely what the cars are doing that give you a "real-time" mpg readout.