OT: Car speedometer accuracy

[Another John]

Got a new car last week (3-yr-old Yeti). When I was using the satnav I noticed that the car's speedo was consistently 2 to 5 mph higher than the speed that the satnav showed.

I know that cars' speedos tend to be a bit inaccurate (even a 63-reg Yeti??). And as for the stanav, it needs to be seeing enough satellites (I guess) to get the speed correct.

So does anyone know which is likely to be *more* accurate? Speaking as someone who's just been nicked for going 35 in a 30 zone, I'd be interested to know.

Cheers John

[Bob Minchin]

Sat nav will likely be correct Car Speedos must NOT underestimate and so to be sure they tend to over read by a few % in order to meet the specification.

You notice this in the Average speed zones, if you set the cruise control from the satnav, you are continually chasing up the arse of the car in front who is driving on his speedo

[Terry Casey]

My previous Focus (02 Reg) was also consistently 'fast' compared with the SatNav whereas my latest one (61 Reg) is virtually spot on - to the extent that I have to keep reminding myself not to make allowances for the 5mph difference!

Most cars read high - I think it is because they are not legally allowed to read slow and most manufacturers play safe.

One experiment you can try when approaching a speed camera: stick to the limit as displayed by the SatNav and watch the other traffic. Other drivers who rely on their SatNav will be doing the same speed as you whereas those who rely on the speedo tend to drop back about 5 mph ...

I haven't tried it recently but it was something I noticed when I got my first SatNav.

[simon mitchelmore]

Agree with Terry, esp round average speed cameras, on that long 50mph on the M1 it makes a big difference over then 9 miles affected.

Also the old type non-digital speedo's suffer loss of magnetic strength say > 20 years which causes them to eventually read slower than true so guess manufacturers are mindful of that and their potential liability as well, so set them high.

[Michael Chare]

The issue you raise is often mentioned. Speedometers over stating the speed is a feature of every car I have owned. The problem gets worse as the car tyres wear out.

The GPS will probably be more accurate. (Due to the random nature of the positions they record.)

A long time ago when the equipment was large enough for me to see the meter, I once watched the police at work on a London street with a

30mph limit. I think they only stopped vehicles where the speed was 37mph or more.

[polygonum]

Not if you have adaptive cruise control...

(You are still behind them travelling slower than the limit, but you keep a decent distance from their arse.)

[bert]

Must not over read by more than 10%

[F Murtz]

The all read that way, the only vehicle I have that is on the knocker is my ex police BMW bike.

[Bill Wright]

The satnav will be about right. Cars always exaggerate a bit. Measure a mile accurately on the motorway using the OS map then time yourself as you drive along it.

Bill

[harry]

You can check your speedometer on the motorway. At 60mph, you pass a milepost every sixty seconds.

[Brian Gaff]

Assuming that the speedo works from a mechanical linkage of some kind to a wheel and nobody has fitted the wrong tyres or anything else odd has happened, it sould be very accurate. The speed from satnave surely has to be more crude due to the different satalites in use, and how many and at what angle they are. I'd imagine that this is averaged over a fairly short time but not as immediate as a mechanical system would be. I did think some form of doppler radar might work, but the question is where do you aim it that would be consistently detected? Brian

[Tim Watts]

You were unlucky! You didn't ask him if his head went to the top of his helmet or somesuch, did you? ;)

IME I would assume GPS is more accurate, however, a controlled trial speed run on a motorway between 2 fixed points would verify which one (if either) was more correct.

[The Natural Philosopher]

All I can say is that this is approximately the exact reversal of the truth.

A mechanical speedo is hard pushed to get down to +- 5%. Without constant calibration and adjustment.

GPS does its own constant calibration and adjustment. Against the most accurate time keeping technology known to man

[Tim+]

I don't think they've worked like that for a very long time!

Tim

[Harry Bloomfield]

Another John formulated on Sunday :

[Harry Bloomfield]

Another John explained :

Your Satnav will generally be spot on, but with some exceptions. Your speedos hardware will have the ability to be spot on, but car manufacturers deliberately program them so as to over read your speed - so as to always be legal. They are not legally allowed to under read speed, but are allowed to over read by a margin, so they are programmed to always be within this margin.

Exceptions are, vehicles produced for the emergency services, where the speedos will be set to be spot on. I have had a few of these.

[Huge]

Utter, complete and total tripe.

[Harry Bloomfield]

Huge was thinking very hard :

I think you need to explain your comment.

I have had GPS in my car for 20+ years, long before they became satnavs. I have found them spot on within the +/- 1 digit consistently at a steady speed and assuming reasonable reception on a reasonably straight road.

My present car has access to three speedos - the satnav, the usual analogue and a digital one. The satnav and digital (derived from the same system as the analogue) always read accurately and in agreement. The analogue always reads that few percent high, plus one.

[Peter Parry]

The GPS.

Virtually all car speedometers over read. Speedometer accuracy is subject to the United Nations regulation 39.

This is embodied in UK law by the Road Vehicles (Construction and Use) Regulations 1986, as amended,which allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39.

Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than110 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.

[NY]

I would trust the GPS speed more than the car speedo.

By comparison with GPS, my speedo seems to over-read consistently by about 5 mph for all speeds from about 20-70 mph - so I know that I'm safe at an indicated speed of 75 on a motorway or for indicated 35 in a 30 zone. Usually I tend to forget about this, so I drive at indicated 30, 60, 70 etc which means I'm really doing 25, 55, 65 etc - which errs on the side of caution.