Which is more accurate, the speed on my sat nav or on the car speedometer?

Question

I think I was flashed by a speed camera on a motorway contraflow with a speed limit of 50mph, but when I looked at my sat nav it said I was travelling at 50mph and the speedo said 55mph. Which one should I rely on?

Answer 1

your car is correct not the sat nav and the law says you can allow 10% + 3 to the speed and just coz you have been flashed dosn't mean you are going to suffer. There may not have even been a film in the camera, i would also asked when it was last colaborated as well and they can't do everyone anyhow, they can only do a certain percentage of those flashed and hopefully you are one of the lucky ones!!

Answer 2

In the UK, at least, car speedometers are allowed to be up to 10% wrong, and the error must result in an over-read, never under. So if your speedo read 55, you could well have been doing a true 50, as your satnav said.

Anyway, the cameras are usually set with a "margin" built in, commonly 10% + 2 mph, so it would have triggered at a true 57mph. It was probably flashing someone else.

Be aware, though, that these days some speed-check equipment is set to use an average speed over a set distance, not a snapshot, and this is often employed in roadworks & contraflows.

Answer 3

We just answered this question the other day.

Your GPS will be more accurate. This is because automakers set speedometers to read slightly slower than your actual speed, for what I would think (maybe not, considering some of the people here) would be an obvious reason: You don't want to be going faster than you think you are and get a ticket for speeding.

Remember that your speedometer's reading will be influenced by the size of your ties in addition to the built-in speed cushion. A GPS receiver measures speed by constantly averaging the time it takes you to travel between two sample points. Your speedometer isn't that smart.

Porter... says your speedometer is more accurate, but then he tells you why the GPS is more accurate. And the other guy is flat-out wrong when he says the GPS knows distances only as the crow flies.

Answer 4

Unless you suspect that your vehicle has a severely defective speedometer, use it. While I do not doubt or argue that your GPS speed IS probably fairly accurate, I would not rely on it for your true roadspeed. Here's why: In the US ( I assume you are either in the UK or Australia by the term "motorway") auto manufacturers set the speedometer approximately 3 Mph FASTER than the actual speed of the car. The main factor is insurance, and limiting how fast you are actually driving. If that holds true in the UK, Australia or even Canada remains to be another question. Another strong position is that a traffic court would look only at the original equipment on the vehicle... in short, until the laws catch up to the technology, use your car's speedo. Hope this helps some.

Answer 5

The GPS system was developed for use by the military to track ground troop movements and also aircraft etc,. Your GPS is more accurate than your car speedo Depending on your car type and make you will find that if your car speedo is showing 50mph your GPS will show you are doing 46mph If you are worried about being flashed by the camera check out this site http://www.speed-trap.co.uk/

Answer 6

The GPS location is granular, so it can only give useful averages over long distances. The trouble with that is:
a) if you don't hold constant speed, it'll give an average speed.
b) it can't know the road has curved. It'll calculate based on distance that the crow flies, your actual speed will be faster.

You might use a GPS for a one-time calibration over a nice long dead-straight dead-flat piece of road where you hold speed spot on constant. However you'd be better off using a stopwatch and mileposts (or whatever they have over there.)

Answer 7

your sat nav is more reliable because when i was going past a speed camera my sat nav said 30 mph and so did the camera but my speedo said 36 mph

Answer 8

The sat-nav is accurate, but it measures speed assuming you're on the level. It also only updates every 10 seconds or so. Your real speed at the flash was something in between. You need something like a mile of level straight road to do a proper comparison.

Answer 9

The speedo.

Answer 10

your car speedo, that's what its there for.