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I was talking to my son today about a random theory of a friend of mine. His theory was that speedometers are typically set so that 12 o' clock is the optimal speed for freeway driving in that particular car. So a car with a more powerful engine might put 70mph at the top, while a car with a weaker engine would put say 55mph up at the top.

So... how are speedometer dial ranges set for a given car? Is the max speed set to the fastest the car can reasonably go? Is the preferred freeway speed up at the top of the dial? It's got me curious now -- I know there's wide variations between makes, but there's probably some method to how speed is displayed to the driver.

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There are many variations, and no consistent method across manufacturers. That said, your theory is not correct - you are more likely to get the legal highway speed somewhere near the top because that is an area where the eye can easily see variation.

Even within manufacturers there seems to be a lack of consistency. Where they use the same clocks across multiple models, then obviously the dials will look the same or similar.

The max speed on a speedo is always going to be higher than the max speed of the car by default - where clocks are used across models, the max speed will be higher than the fastest model will drive so the bottom spec model using that hardware may seem to have a lot of headroom on the speed display.

Additionally, for cars that are known to have a strong modding scene, you often find speedos have more headroom - as an example, my Subaru has a speedo that goes up to 240mph, despite the fact that it is not capable of going above 190mph without some serious work.

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  • I know some WRX models are fast out of the box but 190mph or 190kph?? – mikes Sep 30 '14 at 22:44
  • 190 mph. Mine may be slightly non-standard - imported then upgraded by Litchfield before they sell them in UK. – Rory Alsop Oct 1 '14 at 6:33
  • wish they sold them like that in the U.S – mikes Oct 1 '14 at 10:25
  • I believe it if you're pushing 400+ bhp. I can't imagine driving that fast with the stock aero, though. I've felt the back end of my little wagon getting scarily light at fast but not that fast speeds. – Bob Cross Oct 1 '14 at 21:58
  • Yup - just over 400bhp currently in my Forester STi :-) – Rory Alsop Oct 1 '14 at 22:07

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