The speedometer shows the movement of a car: Not the spinning of wheels

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

14 January 2007

I have a policy of not repeating a subject in these columns and, over the last four years of writing, I have adhered to it. However, one issue has been asked by many readers and I think I should break rule – just this once. It is to do with the speed of cars and size of wheels.  Zachariah Kariuki is one of the people who have raised the matter.

He says that he has been arguing with his friends regarding this matter and adds; “Suppose there are two cars, one with small wheels (say, a Mercedes) and the other with big wheels (say, a Range Rover). Both are to travel over a distance of, say 100km at the same speed, which of the two would reach the destination first?”

This argument arises only because “speed” is not clearly understood. In simple terms, speed is the distance that is travelled in a given amount of time. It is most commonly expressed in kilometres per hour (km/h), other units can also be used; for example, metres per second, miles per hour, kilometres per day (for people walking long distances) etc.

So, if the two cars were travelling at say, 50km/h, they would both take exactly two hours to reach the destination 100km away. It appears that Zachariah or his friends think that the speed shown on a car’s speedometer is the rate of rotation of the wheels. This is not the case. The odometer displays the actual movement of the whole vehicle – not the spinning of tyres.

A car’s speedometer tells the driver how many kilometres he will travel in hour if he maintained that rate of motion. Another vehicle with bigger tyres and showing the same speed on the Speedo will not travel any faster!

However, if you fitted larger tyres to your existing car, then the actual speed of motion will be higher than that shown on its odometer. The explanation for this was discussed in these columns last year and I won’t go back to it.

However, Alex Kimani adds another dimension to this matter. He says: “We know that a mile is greater than a kilometre (1mile=1.6km). Say we have two cars, one moving at 200mph and another at 200kph, does it mean that the former vehicle will travel faster?”

To get the answer, we find out what distance each of these cars will have travelled in one hour. At 200mph, the first car will cover 200 miles. This is equal to about 320km. the second vehicle will only do 200km. Clearly, the one doing 200mph is faster. Quod Erat Demonstrandum.