savnat speed accuracy

Trust whichever one is higher

GPS speed accuracy will usually be better than +/- 0.5MPH for a stand alone instrument. Your car speedometer accuracy will be within

+0 and +10% of the true speed. So at 70MPH your GPS will display somewhere between 69.5 and 70.5MPH and your speedometer somewhere between 70 and 77 MPH.

Except that there's now a European Directive on speedo accuracy which allows

+0/+10% plus a further 10kph (6.2 mph). So at a true 30 mph a speedo could actually legally be showing 39 mph which is crazy IMO.

At a true 70 mph it could be showing 83 mph.

I think the manufacturers still abide by the +10% rule though with an average error of about +6% to +7% according to measurements I've taken over the years.

Pragmatically that's the 'safe' solution.

The GPS will be at it's most accurate *if* you are on a long straight reasonable flat road.

At what speed or range of speeds are you getting 10% difference?

That'll explain all the numpties driving everywhere at 24mph then !

Your comment explains the tailgaters when my car is at ~28 mph...

Is that an indicated ~28mph or actual?

[pedant]

Neither. GPS isn't actual...

My speedometer tends to read 2 mph higher than GPS, but neither is going to be accurate... :)

[/pedant]

The GPS is almost certainly accurate or it wouldn't work at all. However it's got a very slow refresh rate so it might not be measuring what you want to know.

The one on the car. If you stick to it, providing you've not put larger diameter tyres on, you'll be at or below the limit.

Not as I understand. It's accurate to a certain distance, but this varies. Any change in vector influences the accuracy of the GPS speedometer.

It's accurate to the distance you've travelled between the refreshes. If you're going round a corner then it'll underestimate your speed but then it won't be constant anyway. In almost any situation where it's refreshing more often than you're changing speed then it's both accurate & will let you "calibrate" your normal speedo. Whicjh is nearly always at least 5% out.

And as I said, what you want to know is not necessarily what it's measuring.

My understandings always been that the faster you go, the more accurate the GPS speed is, due to the distance between updates being higher, allowing for more accurate calculations.

Regardless, my speedo is only about 2-3% out, and that's using two different GPS units, and sitting behind lorries.

Both these explanations are wrong. GPS receivers calculate velocity instantaneously from the doppler shift of the satellite carrier signal. They do not measure time between points on the ground. There will be a small degradation of display accuracy if you indulge in _very_ tight turns due to the processing in small receivers but it is not significant.

How would they differentiate the doppler shift of the carrier due to the vehicle moving versus the satellite moving?

Actually that's not difficult, however I'm unconvinced that's how the commercial receivers do it or the refresh rate would be way higher.

FWIW I have compared 3 GPS's [1] side_by_side and they have all displayed the same speed +_ update errors (typically .1 mph / .5 sec) and various combinations of those three against other makes / models and they have all compared, very very closely. Also FWIW they were all the same make (Garmin) but may not have all had the same electronics.

The one done over the longest distance was my Garmin GPS V against whatever they use in the AA trucks now days from Leek to London (~175 miles). Over the entire trip they both showed identical speeds and distance to next / final etc.

The 2L Sierra estate read 7mph slow at 70 mph the Rover 218SD is nearly spot on at all speeds. Her 1.4i Astra is quite slow reading across the board (so that also explained tailgaters at her indicated

30 mph) :-(

It's interesting when passing through cameras on say a 50 mph to see those who carry on through at a (GPS indicated) 50 mph and those who brake and slow to ~45. Am I right in thinking the camera would typically be set to trigger at the speed limit + 10% plus 2 mph or something? So on a 50 mpg road that would be likely to trigger at an actual 57 mph so an indicated 65+ or something for some people? [2]

All the best ..

T i m

[1] Garmin GPS V, Quest, 2610 versus TomTom in a PDA, a Navman and a couple of built in systems. [2] I have no intention of testing this . Last speeding ticket was in 1978 when 'Exceeding the speed limit for a goods vehicle', 63mph as confirmed by the VASCAR in the Rover SD1 I'd seen following me for a few miles, in a 70 limit in a Moggy Minor Van. (The offence code given was an SP30 when I believe it should have been an SP10?)

Isn't the satellite supposed to be stationary with respect to its position above the earth?

No. Geostationary satellites must be in Clark orbit, directly above the equator. From there, there'd be no GPS coverage at the poles (from about 80º latitude IIRC).

John