Testing a speedometer

[mrmr96]

Hi,

I'm due to fit a new hall effect speedo to my road car. It works by you inputting the rolling circumference of a tyre and it then counts the revolutions to work out speed.

I want to be able to verify the accuracy before I trust it while driving past speed cameras etc!! But my question is what's the best way to test the calibration?

I had a couple of ideas:
1 - Check via GPS. Plus point is it's free (got a TomTom). Downside is that I don't 100% trust GPS speed.
2 - Check on a rolling road. Plus point it should be super accurate (shouldn't it?). Downside is that it could well be expensive to get time on a RR.
3 - Check against a measured mile. Plus point is that it's free. Downside is that I don't know where there are any near me, plus it measures an average so I need to drive steadily to get a decent check.

Anyone got any other ideas? (e.g. is there some kind test I can put the car in for? Could I take it to an SVA centre for example?) Or has anyone got any comments regards the three I've come up with?

Many thanks,

[Ordinary Bloke]

I have some experience of tyre-diameter speedo's, and they're notoriously inaccurate because the shape of the tyre changes with speed, wear, pressure and temperature. GPS speed is actually pretty accurate in my opinion, so I'd go with TomTom speed and recalibrate at different temperatures.

NB: Actually, all car speedo's are pretty inaccurate, for the same reasons.

[Pigeon]

For the measured distance there are marker posts every 100m down the side of dual carriageways and motorways.

[Steve_D]

Your TomTom will give a very acurate reading but you do need to do it on an open road to ensure that TT can see as many satelites as possible so no cuttings or tree lined sections.

The rules for production car speedos require them to not under read so manufacturers have to design them to over-read to cope with manufacturing tolerances. If you have a TT you will already know that most cars you take it in are reading about 4mph fast compared to TT.

Steve

[BB-Q]

Pigeon said:

For the measured distance there are marker posts every 100m down the side of dual carriageways and motorways.

I was under the impression that they're at a tenth of a mile? Given that we don't use metric in any other measurements on our road network?

[SplatSpeed]

my tom tom is 100% accurate to my monaros speed

[tempus]

Put car on a towing dolly, extend speedo wiring into back of tow car,head for motorway and sit in back and adjust at set intervals,works for me.Tempus

[BB-Q]

My Mitac reads 1-2mph under one of those red smiley displays you get at the side of the road. My speedo is pretty optimistic.

[Pigeon]

BB-Q said:

Pigeon said:

For the measured distance there are marker posts every 100m down the side of dual carriageways and motorways.

I was under the impression that they're at a tenth of a mile? Given that we don't use metric in any other measurements on our road network?

All the maintenance and stuff is metric, it's only the signposts which aren't. And I think if they were 1/10 of a mile apart I'd have noticed some horrendously far-out speedos