So Just What Miles Per Gallon Can a Car Get, Exactly?

I was wondering today about cars, and the fact that mileage has remained pretty constant for many, many years, despite convertors and fuel injection and lighter alloys and many more things. It was brought home to me when I read about a BMW keeping up with a Prius, which sounded counter-intuitive.

It’s easy to suggest conspiracies and companies limiting our mileage – but is the problem more a matter of physics?

  • We do need a certain amount of comfort while we drive, and that translates into structural weight and the energy to move it; otherwise, a moped would be everyone’s choice.
  • Cars have to overcome friction when moving – from the air, from the moving parts (even with bearings) – all of which requires constant energy.
  • Nobody likes to talk about the inefficient internal combustion engine, but it needs energy to move itself, as well as the car, and wastes quite a bit as heat.

So taking this into account, what’s the verdict? Rather than fiddle with math here, you can grab a calculator and visit this page for number-crunching; however, it basically summarizes what is possible from a physics standpoint:

  • In a perfect world, and a flat road, the energy to overcome resistance would get a car about 400mpg. But that’s in neutral – think of the energy involved in pushing a car in neutral on a flat road and that’s what you’re looking at. A gallon of gas contains about 132,000,000 joules of energy, so that’s the upper limit with gas.
  • Include the engine in the equation, and the mileage goes way down to 80mpg. Some energy is wasted as heat; some is needed to power attachments (air conditioning compressor, power steering and transmission pumps, alternator, etc.) and a great deal is used to move the motor itself. A motor needs a lot of energy to overcome its own friction – and it’s this friction that reduces the efficiency.

So then is 80mpg a reality? Of course, the article was back of the envelope calculating and personal testing, so it’s a rough estimate. However, until we get a better engine, 100mpg cars are still not a minimal, but a hopeful upper limit to vehicles. Tweaking will add incrementally, but with the engine itself the problem, huge gains are not likely.

Does this mean hydrogen or electric to the rescue? By avoiding the heat issue and high internal friction they improve the possibility, but they are less convenient and portable, unfortunately. Putting a gallon of gas in the tank is a familiar scene, and a common one – try a cross country trip in an electric or hydrogen vehicle to see the huge infrastructure issue ahead of us using those.

Still, it would be nice if people paid attention to the math – such low mileage should have been left behind in the twentieth century…

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