How accurate is the speed read out on GPS devices? I Have just got a tomtom one XL and according to it my speedo is out by about 7mph, as its hardly 'real time' and only updates every few seconds is it just a pointless gimmick?

More accurate than a speedo (as it calculates the speed from outside the car) but not 100% accurate because it is always working out an "average" between co-ordinates.

Chris

How accurate is the speed read out on GPS devices? I Have just got a tomtom one XL and according to it my speedo is out by about 7mph, as its hardly 'real time' and only updates every few seconds is it just a pointless gimmick?

Out by 7mph at what speed? It varies.

at 50mph the tomtom says im doing about 43

Just to be sure, find a straight road somewhere outside populated places (to avoid errors induced by houses/buildings/mountains/trees...) and try to keep up the steady speed. In this conditions, gps speed is very accurate (please see manufacturers specifications for exact speed accuracy).

It's pretty normal for speedo to show less then actual speed. Also, if you have changed tyre size, it would affect the speed indicated by speedo related to real speed.

Don't rely on GPS for an accurate speed.

It can be out by 5mph. I'm not betting my license on that!

ive found with with most vehicles that the gps shows a speed of 5% less than what your speedo shows and i know car speedos are set slightly higher than what you are actually doing so at 60 mph on car speedo, gps will show about 57mph.

car speedos are set slightly higher than what you are actually doing so at 60 mph on car speedo, gps will show about 57mph.

Yes, you're right. It's my mistake in previous post

This (speedo compared with real speed) is done to ensure there is a little extra speed before getting a ticket and also to minimize problems with speed camera's accuracy.