GPS Speed vs Car speedo

[stg]

Just bought a road angel 2 to 'help me be aware of blackspots and schools'.

However noticed that besides the obvious lag for the GPS speed relative to actual, even at a constant speed for 1 minute the GPS speed returns a value 4-5 mph lower than the actual car.

I will always go with the car speed, however interested to know which is more 'accurate'.

[DennisTheMenace]

The GPS is 100% accurate , your car speedo will underread by anything up to 10% at a genuine 100mph my car speedo reads 105 .

[Joe911]

GPS speeds are supposed to be within about a tenth of a MPH, I think.

Car speedos always read (by law) 0 to 10% over, I believe.

My 911 reads 160 at an actual 155 - while the BMW reads about 6 mph over at 90.


>> Edited by Joe911 on Wednesday 8th December 18:10

[robbieduncan]

GPS is probably correct. Car speedos can be read up to 10% over but never under by law.

[fish]

T350c was dead on to GPS to the mph. Lotus overreads by 4-6mph dependant on speed.

[Carnage]

I used to work for a motorbike magazine. When speed testing bikes, it was common for speedos to overread to Blair-like proportions. At a genuine 155mph, radar verified, it was quite usual to see 170mph plus on the bike.

I;m sure the technology must be similar for cars - sensor off the gearbox or a cable, as I recall.

[-DeaDLocK-]

When Autocar lists standing start acceleration times in their comperehensive road tests, they always show the indicated speed for any given true speed.

So, the average car will have a true speed of 60mph but indicate 62mph, a true speed of 100mph but 108mph on the clock etc.

You get used to seeing this discrepancy across the road tests, and the different figures listed side by side in the table become the norm.

I mention this because one of the road tests they did stand out from the rest - the McLaren F1. True speed 60mph - indicated speed 60mph. True speed 100mph - indicated speed 100mph.

From what I understand it's no great feat of engineering to accurately calibrate a speedometer. But manufacturers don't bother due to a gentleman's agreement to always overread and simply to implement a margin or error as a "just-in-case" cop-out.

D

[pdV6]

DennisTheMenace said:

The GPS is 100% accurate

Need to add to that:

"... when travelling at constant speed in a straight line with good satellite coverage"

in order for it to be true.

[flat_steve]

pdV6 said:

DennisTheMenace said: The GPS is 100% accurate Need to add to that: "... when travelling at constant speed in a straight line with good satellite coverage" in order for it to be true.

Yeah, I remember testing a GPS jacket for an iPaq pocketPC that insisted the building we were in was moving at 2mph down the road...

[Don]

I compared my Boxster S speedo to the GPS unit.

Up to about 100mph the car is absolutely spot on.

At a VMAX event I went through the trap at what I thought was 152 indicated but got 148mph.

I think I lifted slightly prior to going through the trap..but can't be sure. Even so at that speed 4mph out isn't bad at all...

[mybrainhurts]

My Origin b2 and TomTom nav display identical speeds, from different receivers, of course.

When they say 70, the car says nearly 80.