Is GPS MPH accurate

[hindle8907]

Just been for a ride out and tracked the route on my GPS device.
it shows me where i did my max speed on the software and this is 2 mph over what was indicated on my speedo . but i thought speedos over read ? not under ?
cheers.

[Wideboy]

no they are not accurate

[DarrenSV650S]

How long were you riding at your max speed? You need to be at that speed for a few seconds for the gps to register it I think

Sometimes read more sometimes read less. I'm sure there's a use for it for average mph / miles covered but i wouldn't trust it for speed cameras or whatever.

My GPS Tracker is always reading less than the speedo.

e.g If i could get too 100MPH on the speed then the GPS would probably show about 93-94 MPH

[hindle8907]

you would think the GPS would under read yeah ... i didnt think it would over read .

[DarrenSV650S]

What gps do you have? Also, you didn't answer my first question

Originally Posted by hindle8907

Just been for a ride out and tracked the route on my GPS device.
it shows me where i did my max speed on the software and this is 2 mph over what was indicated on my speedo . but i thought speedos over read ? not under ?
cheers.

I used to work with an ex- ops manager for VDO, who make speedos etc for loads of manufacturers, he told me most clocks were approx 5 mile over when they sent them out prior to fitting on the track.
You think about it, if everybody had a speedo reading over its the best way of reducing speed.
Ive tested this theory, the signs on the street that flash your speed up are always 3-5 mph out to what my speedo says.
Things like tyre wear can affect the true accuracy of the speedo as they wear, the circumference / diameter of the wheel reduces in size meaning every revolution of the wheel is not travelling as far.
Dont police have to have their speedos calibrated regularly?

My tom tom seems more accurate to these speed signs when i have it on while going past them than my speedo

What is speed? Distance divided by the time to travel that distance. A GPS is very good at measuring location (distance) and time so therefore it should be accurate. I have used TomTom on my iPAQ since 2003 and it's always been accurate for me. Modern speedos seem to all lean towards the top end of 10% over.

the royal air force do not believe gps systems are accurate, and their technical department is bae systems

i think there have been a few threads on here about this subject, the majority have the view and knowledge that they are not accurate.