Relationship between engine capacity and vehicle speed

A vehicle with large tyres travels faster than one with small ones even when the two are doing the same speed, say 80km/h?

Saturday March 12 2016

The key in the explanation is simply that the stated speed is not the rate of rotation of the wheels but the distance the whole vehicle would cover in one hour – no wonder we say so many kilometres per hour! PHOTO | FILE

The key in the explanation is simply that the stated speed is not the rate of rotation of the wheels but the distance the whole vehicle would cover in one hour – no wonder we say so many kilometres per hour! PHOTO | FILE 

By MUNGAI KIHANYA
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A few years ago, I dedicated several consecutive weeks in this series to debunking the widely held notion that a vehicle with large tyres travels faster than one with small ones even when the two are doing the same speed, say 80km/h. The key in the explanation is simply that the stated speed is not the rate of rotation of the wheels but the distance the whole vehicle would cover in one hour – no wonder we say so many kilometres per hour!

After all that effort, it has come to my attention that there is another “theory” among certain motoring enthusiasts: That a vehicle with a more powerful engine travels faster than a weaker one even when the two are doing the same speed.

This came to me via Ali Bofulo who says that he had an argument with his in-laws and when he wasn’t able to convince them the truth, he stopped arguing lest they decide to take back their daughter!

According to the story related by one of Ali’s in-laws, a small car was caught by a speed camera in Dubai for exceeding 120km/h yet, at the same time, a bigger vehicle was let off even though it had just overtaken the smaller one. “Small” and “big” here is in reference to the power of engines.

Ali tried to convince them that any two cars doing the same speed will cover the same distance in equal times but they could hear none of that. So he gave up and sent an S.O.S to me.

MEANING OF SPEED

My first reaction to the story was the quote I got from my math teacher: “Complete, utter, absolute, diabolical nonsense”. 120km/h is 120km/h whether you are driving a 69-horsepower, 997cc Vitz or a 550hp, 3,855cc Ferrari – both will cover equal distances in equal time and none can overtake the other.

Anyone saying anything different needs to go back to primary school (Class 4, I think) and learn the meaning of the word “speed”.

Of course, the Ferrari will pick up speed more quickly than the Vitz – that’s called acceleration and it is taught somewhere in lower secondary school – but when the two reach 120km/h, none can overtake the other.

Now, I must also clarify one other point: power alone will not make the vehicle pick up speed quickly. Think about it: the 550hp-Ferrari can probably reach 100km/h in less than five seconds, but a 700hp-Scania V8 prime mover will need close to 20 seconds to get to the same speed. In fact, even the 69hp-Vitz will get to 100km/h much quicker than the Scania.

Two important factors are at play: the gearing ratios and the mass of the vehicles. Because of these, the power rating of an engine is a completely useless quantity to a motorist! It tells you nothing about the motion of the car. It has only one use – marketing! Marketers know that men (and let’s be honest, most women consult men about vehicle choices) are impressed by large numbers even when they don’t understand what the figures mean.

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