Wheels and Engine Rotation

Do the wheels of a car turn at the same rate as the engine?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

05 March 2023

Josephat Munguti wants to know whether the wheels of a car rotate at the same rate as the rotation of the engine. “When I am driving at 100km/h, the engine rotation counter in my car shows points to just under 2,000rpm. Does this mean the wheels are also running at 2,000rpm?” I asked Josephat to measure the diameter of his tyres to help with the calculation. He wrote back saying that they are 70cm.

With a diameter of 70cm, the circumference of the tyres is about 220cm. This means that every time the wheels make one complete rotation, the car moves forward by 220cm. The question then is: when the car is at 100km/h, how many rotations do the tyres make per hour?

To find the answer, we re-phrase the question thus: how many rotations do the tyres make when the car travels 100km? First, we convert 100km to centimetres. This comes to 100 x 1,000 x 100 = 10,000,000cm.

Next; we divide this by the distance traveled per wheel rotation. That is, 10,000,000cm divided by 220cm. The answer is 45,454 rotations. Now, since we are working with speed in km/h, it is obvious that this is the rate of rotation per hour. But the engine rotation counter gives reading of rotations per minute - rpm.

Therefore, we need to convert our result from “per hour” to per minute”. In other words, we need to divide the 45,454 by 60. The answer is: 757rpm. That is, in Josephat’s car, the wheels rotate slower than the engine – even when it is driving in its top gear. At lower gears, the tyres rotate much slower for the same 2,000rpm. Indeed, when the car is acceleration from rest, the engine sounds much louder than when it is at cruising speed.

This brings out an important, but normally unappreciated fact: car engines are rather weak! Without gearing, they cannot move the car. In the case of Josephat’s car, the engine needs to turn 2.64 times for the wheels to make one rotation (2,000 divided by 757).

Finally, let me reiterate one point that I suspect will be raised: vehicles with large wheels (lorries, buses etc.) DO NOT move slower than those with smaller wheels (cars) when their speedometers are showing the same value. Whether it has big or small wheels, 100km/h is 100km/h! Yes; the bigger wheels will turn more slowly, but the speed of the vehicle is the same!