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Lindytoes: I then checked the RPM that my car was producing at each speed and the gearing. At 55 MPH, the transmission was in the same overdrive gear as at 75 MPH. Next I looked at the RPMs, I was at 1800 at 55 and 2100 at 75 MPH. This was the only major difference. I then looked at the power curve on my engine and saw that my main horsepower does not flatten until I hit around 2000 RPM, which is the constant power on my car.

I then looked at your Malibu and saw the curve. I would say that this is the reason that you are getting worse gas mileage at 75 vs 55. Your car is way, way underpowered. This is the same as in Europe. I have visited there dozens of times and was always amazed that I could rent an Audi A4 or Seat or whatever, and get 35 MPG at 120 KM = 75 MPH and why that was. The reason is that European cars are built for highway driving, their power curve is set as flat at higher RPMs, therefore they are more efficient at highway speeds - 110-130KM per hour, 65-80 MPH.

The usual internal combustion engine has a full throttle efficiency peak somewhere between the torque peak and the horsepower peak. Most of us drive well below either peak, and almost no one drives at full throttle for any length of time. I don't know anything about part throttle efficiency.

The horsepower required is roughly proportional to the speed cubed, regardless of anything else. The drag is proportional to the speed squared. (For a flat road - obviously a hill requires horsepower to raise the weight of the car up the hill.) These are physical facts.

My Chevy Venture gets around 21 mpg at 80. At 70, it's more like 26. At 60 it's up to 28. (These numbers are based on driving long distances.) I have observed the same thing in several cars.

Your Corvette is running way off of peak horsepower, or even peak torque, at the speeds you mention. Its efficiency is still increasing, even at 85 mph, which may offset the increased drag. Do you know when the efficiency peak is? Not knowing your numbers, the new Corvette boasts 430 HP at 5900 rpm, and peak torque of 424 ft-lb at 4600 rpm, both well above the speeds (and rpm's) you mention. Peak full throttle efficiency would be between these speeds, say 5000 rpm.***

Barring some numerical analysis, I doubt your thoughts on European cars. An Audi A4 is likely the same car here as in Germany. Do you know that the engines are different? (Drive trains are almost surely different. I once bought a Jaguar E-type coupe (stick shift) in Germany. At 100 mph, I could finally shift into 4th gear.

Count Upp
*** Oops! I see this is not your experience, but that of a reader to the blog. So now I feel free to call BS on the lot of them. It is true that inflating the tires and keeping the engine properly tunes will increase gas mileage. It is also true that operating well below the efficiency peak (normal situation) will hurt gas mileage.
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