Do car speedometers show the correct speed?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

20 November 2022

If the traffic police stopped you and accused of speeding, how sure are they that you were actually over the speed limit? Are they certain that their speed-gun measures the correct values? I wondered about this recently while I was driving using Google Map to get the best directions to my destination. I noticed that there was a difference between the speed indicated by the car’s speedometer and that shown on the Google Map.

I slowed down to a steady 40km/h and asked my passenger to check what Google was showing; it was 36km/h. I increased speed to 50km/h and we compared again. Google was saying 47km/h. I became more curious and cross-checked the readings in steps of 10km/h all the way to 100km/h. The results were 56km/h, 65km/h, 76km/h, 85km/h, and 95km/h, respectively.

My first suspicion was that there is a problem with my car’s speedometer – it is an electro-mechanical analogue instrument. About a week later, I happened to ride in a friend’s car and checked the readings again. The results were similar – Google Map showed speeds that were consistently 5km/h lower than those of the speedometer.

At this point, I tried using one of the online speed meters. The results were the same – the car’s speedometer was consistently higher by about 5km/h. Upon further thought, I realised that all online speed meters use the same GPS satellite signals to do their calculations, so they are bound to show the same answer!

So now there is an unanswered question: does the speedometer of a car give the correct reading? This brings back memories of the time when Kenyan TV stations were having a dispute about time-keeping. Viewers had noticed that each station was showing a clock with different readings – with the differences running to a few minutes.

Even though the national broadcaster and/or the national telecommunications company are usually the custodians of national civil time, the dispute between our TV stations ended with the coming of GPS time signals. Perhaps car manufacturers should borrow a leaf and start installing GPS speedometers instead of the electro-mechanical contraptions used today. Since there’d be no link to the gearbox, this would permanently dispel the fallacy that cars with larger wheels move faster than those with smaller ones when the speedometers are showing the same readings.