Driving efficiently: saving fuel and money

Last updated: July 13, 2008.

For every pound's worth of gasoline (petrol) you squirt in your tank, only about 30¢'s (15p's) worth moves you down the road. The rest is wasted heating up the engine, whirring the gears, blowing out the exhaust, running the air conditioning, burning the headlights—and all the rest. In scientific terms, cars are pretty inefficient machines: they use only 15 per cent of the energy they consume doing what you really want them to do. What can you do to get more miles for your money?

Photo: Filling up's getting more expensive and it's never likely to get much cheaper. For the sake of your pocket (and the planet), it makes sense to think hard about fuel efficiency and the way you drive. Photo by Warren Gretz courtesy of US Department of Energy/NREL.

What is efficiency?

Something is efficient if it uses energy well—and some machines are much more efficient than others. A bicycle takes about 90 per cent of the energy your legs supply at the pedals and turns it into kinetic energy (the energy you have when you're speeding down the road). It's a brilliantly efficient machine. A car engine is only about a third as good. Efficiency is the amount of energy something produces divided by the amount it uses, multiplied by 100 to give a percentage. A bicycle is 90 per cent efficient, a car engine just 30 per cent.

How do cars waste energy?

Cars waste energy even before they're built. Add up all the energy a car needs during its lifetime and you'll find 10 per cent is used just getting the thing on the road: 6 per cent goes into making the steel and all the other bits and the other 4 per cent is needed in the car factory to put them together. As you'd expect, the vast majority of the energy a car consumes in its life (90 per cent) goes into the fuel tank in the form of gasoline. But what happens to it then?

A car turns the energy in gas into movement in two separate stages. First, the cylinders in the engine burn the gas and release the energy it contains, but even the best engines do that quite badly. A good gasoline engine is about 30 per cent efficient—so it gets just under a third of the energy out of the gas it burns and wastes the rest. Diesel engines are much better at around 40-45 per cent, while hybrid electric cars (which have gas engines and electric motors and can switch between the two) manage an impressive 60 per cent.

The second stage of propelling the car involves transmitting the energy from the engine to the wheels through a fairly intricate system of gears and drive shafts. There are dozens of moving parts in a car's engine and each one of these wastes a little bit of energy passing power along to the wheels. No matter how well oiled the engine is, each moving shaft or gear rubs against the parts that hold it in place. The force of friction makes it heat up and that wastes energy. Moving parts also make quite a bit of noise—and that's wasted energy too. Only about 40 per cent of the energy that leaves the engine arrives at the wheels; the rest is wasted in the transmission. Add this to the energy wasted in the engine and you get an overall efficiency for cars of about 15 per cent.

Photo: How many more years of oil are there left in the ground? Are gas stations like this slowly heading for extinction? Or, in the future, will they start serving us with biofuels or hydrogen to power fuel cells? Photo by Warren Gretz courtesy of US Department of Energy/NREL.

Where does the energy go?

Car engines work best when they're working hardest, at high speeds and high power. Driving slowly in city traffic, all the stopping and starting means an engine never really gets chance to work efficiently. Energy is constantly being wasted moving the car off from a standstill—and if you've ever pushed a broken-down car, you'll know just how much energy that needs. A typical car weighs 20 times as much as the driver, so a huge amount of energy is needed just to move the metal. No sooner do you get going than you have to step on the brakes, undoing all the engine's hard work. The brakes press against the wheels turning all the kinetic energy you've built up into heat that vanishes into thin air. That's why, at city speeds of less than about 40kph (25mph), a car is working at only a third of its maximum efficiency.

Artwork: Cars work most efficiently in their mid speed range at about 80kph (50mph) or so.

When you're on the open road, the engine becomes more efficient the faster you go—but only up to a point, because another factor quickly takes over: air resistance or "drag". To move a car, you have to push the air in front of it out of the way. A typical car has to move 5.5 tons of air for every mile it travels. (Think of it like a snow-plough, only shifting invisible gas out of the way instead of snow.) The trouble is, the faster you go, the harder it gets: the more you stir up the air and the more energy you have to use pushing against it. At about 80kph (50mph), you're using just over half the car's energy overcoming drag. Speed up to 110kph (70mph) and the fight against drag will use 65 per cent of your energy. Drive at 300kph (185mph) and drag will take 80 per cent of your power.

Some tips for saving fuel and money

People are getting more concerned about the environment and issues like global warming, while the cost of oil keeps on increasing. In the long term, cars are slowly becoming more efficient. But that doesn't help you in the short term: once you've bought a car, you're stuck with it—for a few years at least. What, then, can you do to get more bang for your buck? As the graph up above shows, the best thing you can do to save fuel and money is drive in a mid speed range for as much of the time as possible: don't go too fast or too slow. There are plenty more things you can do to go further on each tank of gas. They're good for the planet, and they save you money too, so it's a win-win!

Which cars are most and least efficient?

Heavy cars waste energy moving the metal; fast cars waste energy pushing the air. The most efficient cars tend to be small, light ones going at relatively modest speeds. This chart shows (roughly) how many barrels of oil a range of cars might guzzle their way through in a typical year:

Credit: Chart by Explainthatstuff.com using energy impact scores shown on the US Department of Energy's Fuel Economy website.

Further reading

• Cutting the cost of driving: In this short (3 minute 39 second) video, Guardian journalist Patrick Collinson learns how to save money by driving more smoothly and efficiently.