How efficient is a car?

The Second Law of Thermodynamics puts an upper limit on the efficiency of a heat engine. As I understand it, the most efficient heat engine is a Carnot cycle engine, named after Sadi Carnot, who worked out the  maximum theoretical efficiency of any heat engine. It’s (1- Tc/Th), where Tc is the temperature of the cold side of the engine, and Th is the temperature of the hot side. Heat flows from the hot to the cold side, and on the way some of it can do useful work.

So how much work can a gasoline engine do?

In real life, friction and other factors reduce the efficiency of the engine. Long years of experience with gasoline engines shows that typically they operate at about 25% efficiency. That is, of every 100 litres of fuel you put in your tank, about 25 litres move the car and what’s in it. The other 75 litres are wasted in the form of exhaust gases, friction, heating the engine, etc.

That’s the efficiency of the engine. It’s not the efficiency of the car.

To calculate the efficiency of the car we need to know the total weight of “the car and what’s in it”. You, the driver, are in it.

Let’s say you are a typical Canadian male and weigh about 200 lbs (90kg). A car itself typically weighs about a ton (2000 lbs, or 900kg). Together, you and the car weigh about 2200 lbs, or 1 tonne (1,000kg). So you weigh about 10% of the total.

So only about 10% of those 25 litres that move the car and you actually move you. Or, thinking about the fuel in the tank, out of 100 litres, 2.5 litres are used to move you down the highway.

That means the overall efficiency of the car as a means of transporting its lone driver is 2.5%.
Since fuel costs money, that means of every $10 you spend, 25 cents will pay for your transportation, and the other $9.75 pay for moving the car, wearing down its parts, and heating the air.

You can increase that efficiency. A lighter car is more efficient, because you make up a larger fraction of the total weight. A car loaded with passengers and their gear is also more efficient, for the same reason. A vehicle that carries a lot of passengers and a lot of gear, like a van or a bus, will be even more efficient. However, with just the driver, a van or bus will be less efficient. A pickup truck, which weighs considerably more than a car, will always be less efficient.

From a selfish point of view, one should buy the lightest car one can use, and use it as little as possible. From a social point of view, one should probably not use a car at all, or only when absolutely necessary.