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[Plomien]

if you want a more accurate speed just use a sat nav

[Ian_D]

You can clock them as I've got a speedo somewhere in the garage from an Omega with 1 mile on the clock! (I think it had 200k+ on it before I took it to bits!).

I seem to think it was very fiddly though...

[tunnie]

if you want a more accurate speed just use a sat nav

Thats actually bad idea, SatNav speed is often off too.....

[the alarming man]

it is only accurate any sat nav on a flat straight road and even then it is a little out as it was built into the sat so the ruskies didnt know where it was/is

[Kevin Wood]

it is only accurate any sat nav on a flat straight road and even then it is a little out as it was built into the sat so the ruskies didnt know where it was/is

They actually turned off selective availability, which deliberately introduced an error for security reasons, 15 years ago. GPS can give a speed signal to an accuracy of 0.1 MPH or so if the signal conditions are good and the direction of travel is constant.

Of course, that assumes your sat nav is showing you GPS speed (calculated by the GPS chipset from signal doppler shifts) and not calculating it by comparinging successive positions reported by the GPS, or using the speed signal reported by the ABS ECU.

[TheBoy]

Accuracy of standard ones is actually quite close - usually about 1-2mph over over the entire range.

Satnavs, most are pretty inaccurate, as they try to time between points.  You can get proper GPS devices that do work it out accurately.

[the alarming man]

Accuracy of standard ones is actually quite close - usually about 1-2mph over over the entire range.

Satnavs, most are pretty inaccurate, as they try to time between points.  You can get proper GPS devices that do work it out accurately.

for a price.. i guess quite a high price

[Kevin Wood]

Accuracy of standard ones is actually quite close - usually about 1-2mph over over the entire range.

Satnavs, most are pretty inaccurate, as they try to time between points.  You can get proper GPS devices that do work it out accurately.

for a price.. i guess quite a high price

No. Any GPS receiver will give you a very accurate speed in the NMEA serial output. What it displays (if it has a display) might be filtered and mucked about with, but will be spot on if you don't change direction or speed and have good signal conditions.

Kevin

[Del Boy]

As to this 'clocking' malarky, it's only an offence to sell a car if you know it's been clocked, and it can only also be an offence if you state an incorrect mileage to be genuine. For example I could buy an Omega tomorrow which has done 200,000 miles, whack it back to 60,000 and make up a service book, but if I state the mileage reading is unknown to be correct then I haven't actually done anything illegal.

[TheBoy]

Accuracy of standard ones is actually quite close - usually about 1-2mph over over the entire range.

Satnavs, most are pretty inaccurate, as they try to time between points.  You can get proper GPS devices that do work it out accurately.

for a price.. i guess quite a high price

No. Any GPS receiver will give you a very accurate speed in the NMEA serial output. What it displays (if it has a display) might be filtered and mucked about with, but will be spot on if you don't change direction or speed and have good signal conditions.

Kevin

To clarify fr those that may not understand what Kevin Wood has said, the GPS antenna reports back to the 'computer' (be it laptop, satnav electronics, speed measuring device etc etc) various things in an industry standard format (well, there are 2 or 3 different formats, though NMEA is by far the most common).  These packets include not only position, but also direction of travel and speed of travel.

The accuracy of all these devices is improved with the number of fixes the antenna/receiver can pick up.

The problem with your gay little tomtoms etc, is because its used in a relatively hostile environment - in a tin box, behind heat refelctive glass, and to make the unit small, a compromised GPS antenna - your signal is less than ideal.  Added to this that the silly little unit is only useful for the last mile anyway, where tall buildings can cause all sorts of signal problems, the software in the devices has to molest the signal.

The likes of Tomtom can make an assumption that you are on a road - we've all driven around new bypasses that aren't on the digital map, and you can be well over 200m from the old road before the daft thing conceeds you are in a field. 

[2woody]

I've recently done a back-to-back test between some pretty expensive UK MoD GPS systems and found that the most accurate was my £79 hand-held Garmin unit