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[Kevin Wood]

Also depends on whether we are at war or not.

For those of you who remember the 1st Gulf war conflict, you may have encountered some very strange sat nav issues as the "spec" of the military satellites were altered to give some strange readings - the reason we were given by the manufacturers at the time was it was to prevent any GPS targeting being used against the US.

So bearing in mind that its also a US system, I think you would be very foolish to trust it, and would recommend using it only as a guideline of speed

A bit later in the war they apparently ran out of military GPS receivers and turned off SA (selective availability) so they could use domestic receivers in the field. I was testing a mapping system for GSM coverage at the time and was amazed to find my traces could be overlaid onto a 25K O/S map and they fitted perfectly.

But yes, the system is primarily a US military one so they call the shots. It's wise to bear that in mind.

In addition, the signal is frequently, and easily, jammed. Quite often testing of this technology is carried out in the UK and Pilots receive prior notification.

Kevin

[kevinminton]

My method: set cruise at "60mph". Use passenger & stopwatch to measure time between a series of 100m marker posts on motorway. Calculate real ground speed.

This shows 60mph on speedo' is 58mph on ground.
However 60mph on info display-thing is 60mph on ground - I have not been able to measure any inaccuracy, bearing in mind the display rounds to 2 sig fig.

70mph on ground is at about 74mph on the speedo'.

[Kevin Wood]

My method: set cruise at "60mph". Use passenger & stopwatch to measure time between a series of 100m marker posts on motorway. Calculate real ground speed.

This shows 60mph on speedo' is 58mph on ground.
However 60mph on info display-thing is 60mph on ground - I have not been able to measure any inaccuracy, bearing in mind the display rounds to 2 sig fig.

70mph on ground is at about 74mph on the speedo'.

Yep, that's a valid point. Average speed displayed on the MID matches exactly what I've measured on a GPS so I suspect it's not set to over-read like the speedo is. Handy to reset it every time you pass an average speed check site.

Kevin

[Lampynoiseboy]

it's actualy not legal to sell a car with an accurate speedo.

there's a formula, but basically expect your car speedo to over-read by up to 6mph at70

That has to be the stupidest thing I've ever read!
Not disputing the truth of it, but it is kinda ridiculous

[hotel21]

My method: set cruise at "60mph". Use passenger & stopwatch to measure time between a series of 100m marker posts on motorway. Calculate real ground speed.

This shows 60mph on speedo' is 58mph on ground.
However 60mph on info display-thing is 60mph on ground - I have not been able to measure any inaccuracy, bearing in mind the display rounds to 2 sig fig.

70mph on ground is at about 74mph on the speedo'.

Remember to add in the conversion from miles/yards per hour into the 100 metre spacings of the marker poles.

Other method I would suggest, similar to the above, is to use the VASCAR markings on motorways and dual carriageways.  They are spaced apart in multiples of one tenth of a mile normally.....   

[Lampynoiseboy]

it's actualy not legal to sell a car with an accurate speedo.

there's a formula, but basically expect your car speedo to over-read by up to 6mph at70

That has to be the stupidest thing I've ever read!
Not disputing the truth of it, but it is kinda ridiculous

And furthermore, what about the ex-plod migs- with calibrated speedos.....surely they're accurate??

[hotel21]

it's actualy not legal to sell a car with an accurate speedo.

there's a formula, but basically expect your car speedo to over-read by up to 6mph at70

That has to be the stupidest thing I've ever read!
Not disputing the truth of it, but it is kinda ridiculous

And furthermore, what about the ex-plod migs- with calibrated speedos.....surely they're accurate??

Bit dusty this one but....

Speedos require to be accurate to within 10% by law.

Car manufacturers want the speedo to read faster than the car is actually going because it strokes your consumer ego.

Police speedos are accurate to within 1% (if calibrated) and that calibration can be screwed by the wear from new tyres with max tread depth to used where tread is towards the minimum.

My force used to change tyres when tread depth came to 4 mm.  Not because the wear screwed the calibrated speedo but because it made the car stick to the road when it had to.