Why do car speedometers get marked higher than their top speeds?

Question

To start, I know that cars can go faster than typical speed limits because if you went on a freeway at 100km/h with a car that had the same top speed, you would essentially be pushing it to the limit constantly, which is not really good for the car.

My question is why do cars have their speedometers marked way higher than they can actually go? For example, my dad’s 2017 Toyota Yaris marks up to 220km/h but it could probably go no more than 150km/h tops, so could they not just mark the speedometer up to 150km/h because it’s not really going to go above that?

EDIT: FYI I live in Australia

In: Engineering

Answer 1

Traditionally old mechanical speedometers were more accurate in the centre of their range. You would often see a speedometer read 10-15 km/h wrong at the low speeds and although it was hard to test I presume this was the case for the high speeds as well. So you had to get a speedometer where the most common cruising speeds were in the middle of the range which meant it could go much higher. There was actually a US law in the early 80s requiring that speeds above 85mph were not marked but the speedometers were still the same as before.

Another reason is that it is much easier to see the top of the instrument dials when driving as this is much closer to the windscreen and therefore your sight line when driving. In some cars you can even see the top of the speedometer in your peripheral vision while looking at the road. So making sure the speed dial is pointing upwards makes it much easier to see. This is more prominent in old racing cars before they switched to digital dials as they would often turn the engine speed dial so the redline was at the top where they could better see it.

But as to the speed I bet you could get your Yaris up to higher speeds then 150 km/h if you tried hard enough. A long steep downhill could get you up to much higher speeds then on flat ground. And with different air pressures, different fuel, different temperatures, etc. the engine would perform differently and you could get more power out of it. Even things like bigger wheels could change the gear ratio and make you go faster.