
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 rephrase 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!

