Engineering Question #2933

Paul Hosking, a 45 year old male from Brisbane, Australia asks on September 15, 2005,

We are having a debate at work that we hope you can settle. The father of one of my workmates has put 18 inch wheels on his car, moving up from 16 inch. He can now travel at 100kph while doing 200rpm less than with the smaller wheels. The debate goes like this. Some think that the larger wheels act as a different gear allowing the car to travel more efficiently. In other words, with larger wheels you travel a little further with each revolution of the driveshaft and will therefore save fuel. Others say that you are moving the same weight at the same speed and therefore will use the same amount of energy. The larger wheel, while travelling further each revolution, is harder to turn and therefore will put a greater load on the engine, causing more fuel to be consumed at lower revs. Fuel consumption should therefore be the same as before. We would really love to know the answer and hope that you will be able to enlighten us.

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The answer

John Jones answered on September 15, 2005

In general, the larger wheels will not improve the car's efficiency or reduce the fuel consumption.

For any engine, you can draw a graph of the engine's efficiency versus its rpm. Running at very low rpm or very high rpm is inefficient; peak efficiency is obtained at some intermediate value, for example, 3,000 rpm. The car's gearbox will be designed so that you can keep the engine at approximately the optimum revs whatever your speed.

The only circumstance under which larger wheels might help would be if you're driving at a speed higher than the maximum speed for which the car was designed -- for example, if you're doing 200 kph in top gear and the engine is at 4,000 rpm. At this top end of the range, the larger wheels would allow you to maintain the same speed while the engine is at a lower rpm, thus allowing the engine to operate closer to its optimum revs. But for lower speeds, the gearbox will allow you to keep close to optimum revs, and the larger wheels will offer no advantage. (In addition, the larger wheels will make it harder to maintain optimum revs at very slow speeds, for example, starting up and climbing steep hills.)

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