The speedometer in my car over-reads. The car, named Bora Horza Gobuchul, has been with me for a while and I remember being on a motorway thinking that I’ll settle at 60m.p.h. as I wasn’t in a rush. After a while I was being overtaken by HGVs and that was quite disturbing given they have a 58m.p.h. speed limit. So, I got a speed application downloaded from the iTunes store and tested my speed using the phones GPS signal.

The result, my speed, according to my phone, was 54m.p.h.

That explained a lot. I had never tested the speedo in the Beast, there never really seemed to be a need to do it. So I set about testing the readings my car was giving me and it turns out that my speedometer over-reads my speed by 10%.

So, if the car is indicating 70m.p.h. then I’m really going 63/64 m.p.h.

My understanding is that speedometers are allowed, legally, to over-read the speed but NEVER under-read the speed. Because of this rule that they are NOT permitted to under-read means that a tolerance HAS to be included for under-reading. It would be costly for manufacturers to produce speedometers that are “exact”.

So, a car indicating a speed of 70m.p.h. could be doing anything in the range of 63 to 70 m.p.h.

Having tested this and observed the traffic around me and used various different speed testing apps I have now adjusted my speed accordingly. This effect could also be deliberate to try and make Prius drivers drive a little slower and therefore save fuel. You use approximately twice the fuel to travel at 70 than you would at 50m.p.h.

Now, here’s the rub:

My car has a “computer” as they used to be called and it tells me average speed, m.p.g. etc. Over the summer I reset this to zero while travelling on a clear motorway. I set the car’s cruise control to a steady speed of 77m.p.h. and reset the trip computer. Once the computer had enough data it flashed up my average speed: 70m.p.h! This continued over the next ten miles of travel before I had to slow down.

It appears that the car’s trip computer reads or reports the correct speed.

This confuses me. The trip computer either uses a different source for its data OR it knows the correct speed from the existing source. I don’t understand why either of these would be designed like that. The car has a mechanical device for reading the speed OR it could use the GPS system but why over complicate things?

I don’t understand my car.