Does not compute

I'm in the process of buying a new car (more on this later) and I'm keen to see how my gas milage will improve. I think the most accurate comparison, of course, will come by recording milage and gas consumption readings over time for similar driving patterns. I'm all set there because I already maintain a milage spreadsheet.

I've purchased 582.495 gallons of fuel this year at a total cost of $1,615.68.

Yes, I'm a dork.

But this morning on my way to work, I had the impulse for a different method of comparison. Although any differences in engine displacement would complicate things, I thought I would record a few RPM / Speed ratios (all in fifth gear) for the Beetle and then see how they compared to the new car. The results were troubling.

RPM	Speed	RPM/MPH
2500	55	45.45
3000	68	44.12
3500	79	44.30

See the problem? My awkward (but I believe useful) metric of Revolutions per Minute per Mile per Hour shows that it's more fuel efficient to drive at 79 mph than at 55. How could this be? I thought President Carter wanted us to drive 55 because it would save energy?

When I was very young, I remember assuming that the body shape of an auto was the dominant performance factor. In my personal experience, a more aerodynamic Matchbox car outperformed a less aerodynamic Matchbox car in all but the most improbable circumstances, i.e., the race was interrupted by a cat or a well aimed rubber band. In the abscence of any contravening data, I assumed the aero-primacy extended to actual automobiles.

So I managed to completely overlook engines. In later years, with the benefits of "real life" driving experience, I started thinking the key was gear ratios. If you've got the perfect gear ratio, you've got the perfect performance. Right?

Well... Kinda, but my six year old self was actually on to something.

From the Wikipedia article on Auto Fuel Economy:

The power to overcome air resistance increases roughly with the cube of the speed. Thus, above about 30 mph (48 km/h), wind resistance becomes a dominant limiting factor. By driving at 45 rather than 65 mph (72 rather than 105 km/h), the power to overcome wind resistance is about one-third, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) increases the power requirement by 2.6 times, and drastically decreases fuel economy. In practice, rather than doubling or halving the fuel economy, the difference is actually closer to 40-50%, since rolling resistance, which is broadly proportional to speed, is also a factor.

So OK, gearing is important and aerodynamics too. But why, for the Beetle, are these readings not showing a significant milage penalty for higher speeds? What am I missing here?

Could it be as simple as bad data points?

It's not like I took the 2500 RPM reading driving uphill pulling a trailer and the 3500 RPM reading going downhill in a hurricane.

Note: If any interesed members of local law enforcement agencies are reading this, note that the 79 mph reading was taken on a closed course rather than on public roadways.