Speedometer

does the mileometer still clock up? If so then the the speedo is faulty or just stuck (give it a whack) If the mileometer stays put then probably the speed sensor has died (or the wiring/connector) but that should show an engine management light fault too.

Buy a GPS, it will have a speed read out !!

Why not work out road speed from the rev counter for now?-

On Tue, 07 Oct 2008 10:20:40 GMT, I waved a wand and this message magically appears in front of Mrcheerful:

Hmm, the mileometer stopped working as well. OK, I'll have the garage check the speed sensor when it goes in for its MOT (due soon).

I've got one but it also broke a few weeks ago - it needs sending back for repair.

On Tue, 07 Oct 2008 11:51:42 +0100, I waved a wand and this message magically appears in front of Chris Bartram:

That's why I was able to do the trip!

In message , Alex Buell writes

No chav nav? Allegedly the speed field from a GPS receiver is accurate to 0.1KM/H.

Probably an electronic one and the sender has a dry joint etc within it. Pretty common fault with these.

Alex Buell explained :

I don't know the car, so just guesses..

That might be one of several possibilities. If the operation of anything else (running of the engine) is affected, check the speed sensor, which is probably some where down by gearbox output shaft.

If just the speedo, it might be stuck against its stop pin (try giving it a gentle tap on top of the dash), or an intermitant connection in the speedo itself/ its plug in - try an harder thump on the dash.

Whoever alleged that to you is wrong.

In principle, using the type of GPS receiver found in satnavs (as opposed to fancy differential GPS like surveyors use for really accurate static positions) you should get a good speed accuracy - manufacturers commonly quote 0.1mph with no explanation.

In practice a moving car GPS is only sampling at most a few times per second, and is applying substantial averaging to the reported positions. The speed measurement obtained actually uses the Doppler shift in the transmitted signal from the satellites to work out the speed, which compensates some for that.

However, the speed is only likely to be anywhere near the quoted accuracy if you have been driving dead straight at a constant speed for a noticeable time (several seconds at least). In real life driving, the satnav has to compensate for corners etc. by using the position estimates (which are very heavily smoothed to give sensible tracks). This smoothing and locking the position to known roads gives a much prettier display but also worsens the speed accuracy.

It's probably better overall than a car speedo, though. As long as you use your own knowledge of what you are doing with the car as well - if you accelerate hard the speed will lag well behind, so you can't use the satnav to accelerate quickly to 70mph then go steady without overshooting.

But of course, it's a manufacturer's spec so it's always going to be a best case scenario and, as such, wrong. Hence 'allegedly'. It's not going to be massively inaccurate though.

I'd be very surprised though if it was less accurate than the average un-calibrated speedo is on the motorway or anywhere you can be lasered or Truvelo'd to any degree of accuracy.

Agreed!

In practise I find I can set the cruise and assuming the bends are not too tight or the cruise looses lock, I get a perfectly steady speed showing on the display. On most normal motorways and fast a roads I can set the cruise to 50, 60, or 70 and it will sit there without budging mile after mile.

The lag is about one second. I can switch the cruise off, let the speed drift, switch it back on and it will come straight back to the same reading on the GPS. I am happy that my own GPS is as precise as might be needed and make good use of it to check the accuracy of the speedos of any alternative vehicles I might drive.

Basically there can be errors due to poor sat positioning and I have heard of errors in some GPS units software, but the errors are fairly obvious when there are any - I know none of my vehicles are capable of

On average it's better, instantaneously it can be far worse.

In message , Duncan Wood writes

Fortunately for me, I currently drive a diesel Focus (actually very nice BTW) and not a tomahawk missile so I suspect I couldn't change my speed quickly enough to lose my license in the time it takes my sat nav to catch up.