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Comment by tsimionescu | Hacker News Reader

I don't understand the argument about "judging your speed". Isn't there a speedometer prominently displayed in every car in the USA as well? You don't have to "judge" anything, just read the number the speedometer shows - is it above or below the speed limit?

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You can't keep a constant eye on both the road and the speedometer. Further, you might have missed the last speed limit change or remember it incorrectly. It's also possible for the speed sensor in your car to be faulty or out of calibration. This happens if you change the size of your wheels/tires significantly without reprogramming the ECU - and that setting isn't made available to the owner of the vehicle, at least not in most cars I'm aware of¹.

In fact, most cars lie to you about the speed - reporting a speed slightly faster than reality. It's a cover-your-ass measure for the car manufacturers because it's illegal to sell a car (in the US, at least) where the speedometer is inaccurate in the other direction, that is, reading slower than actual speed.

1. Some older vehicles, including pre-1996 GM trucks (and probably others from the same era) had the speedometer calibration controlled by a resistor array on a circuit board under the dash, those can be changed with a lot of effort and a soldering iron, or by swapping out the whole circuit board with a different one that matches your tire size + rear end gear ratio.

> In fact, most cars lie to you about the speed - reporting a speed slightly faster than reality. It's a cover-your-ass measure for the car manufacturers because it's illegal to sell a car (in the US, at least) where the speedometer is inaccurate in the other direction, that is, reading slower than actual speed.

Haven't seen that in a long time. Everything I've driven in recent years has a speedometer speed that matches roadside speed sign speed within 1 MPH.

Oh for sure they are not far off, but they always round up to be safe.

My 5-year-old Volvo reports 73-74 mph when GPS tells me that I'm exactly at 70mph. Similarly 21 mph when GPS says 20.

If the law set the speed limit as the hard boundary, with hard punishments for driving at 70, everyone else would drive at 65, occasionally getting close to 68-69.

If the reasonable speed for a road is 70, the legal hard limit could be at 75 to be considerate of this effect. But make 75 the actual hard limit with $200 fines for hitting 75, $1000 fines for hitting 80, and jail time for hitting 85. Make it almost certain to be caught if you hit these hard limits.

I guarantee you fewer people would die on the roads if this is the way things worked.

My experience from all the cars i have driven is that GPS shows 3-5km/h less than the car does. (In Europe)

That's actually a legal requirement (had to google some to get the source): [0]. And a graph: [1]

[0] https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:42...

[1] https://www.reddit.com/r/dataisbeautiful/comments/16mquqv/sp...

The graph seems a bit too much. It says that GPS 110 means ~125 on the odo. Although from personal experience I'd say it's more around a delta of 5 at those speeds, and 3 for lower speeds.

it has to fall somewhere in the shaded area. So if it shows 125, then the actual speed must be between 125 and 110 at worst.

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