No! A car at 60km/h is not slower than a bus at 60km/h

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

26 January 2020

Does a motor vehicle with large wheels move faster that one with small wheels when both are doing the same speed? No. No. No! Speed is distance covered in unit time. It has nothing to do with the size of the tyres.

I discussed this question in great detail in 2007. That’s so long ago that a child who was born on the day the article appeared will be seating for KCPE this year! But, interestingly, four people have asked me about it so far this year.

Some people assume that the speed shown on the dislay inside the car is the rate of rotation of the wheels. Well, that’s not true. Even the gauge reading revolutions per minute (RPM) shows the rotation of the engine – not the wheels.

There is no gadget in the vehicle that measures the rotation of the wheels for that is a useless quantity! What would the driver gain from it? Nothing.

The speed displayed is the number of kilometres the vehicle covers in one hour. Thus, if a small car is doing, say, 60km/h and a big bus is also doing 60km/h, then both will travel over exactly the same distance in equal time. None can overtake the other. Hence, none is faster than the other.

I hope that question is now clear; but our of curiosity, we may want to find out the (useless) rate of rotation of the wheels. Suppose the vehicle is moving at 60km/h. At this speed, it covers one kilometre every minute.

Quick mental sum: speed is 60km/h and there are 60 minutes in one hour; so in one minute, it must be moving just one kilometre.

Now the wheels of a regula car are about 24cm in diametere, so, the circumference is 77cm. This is the distance the car travels when the wheels turn through one rotation.

One kilometre is equal to 1,000 metres; or 100,000cm. Therefore, the wheels turn about 1,300 times when the car moves one for kilometre. But, at a speed of 60km/h, that distance is covered in one minute; so, the tyres are rotating at 1,300RPM.

Bus tyres are about one metre in diameter. We may go through the same steps as we have done for the car to find the RPM of the tyres, but that is not necessary. Since we know that the bigger tyres rotate slower than the small ones, we only need to divide the cars RPM by the ratio of tyre sizes.

The ratio of the diameters is 100cm divided by 24cm; that is 4.17. Therefore, the rate of rotations of the bus tyres is 1300 divided by 4.17. This comes to 312RPM.

This agrees with what we observe on the roads: the tyres of buses and lorries are signoficantly slower than those of cars when all are doing the same speed. Let me reiterate lest I am misquoted: a car at 60km/h is not slower than a bus at 60km/h.