Has anyone checked their vRS speedo against a reputable sat-nav ?

How close/accurate is it (can't be bothered to get in the car and dig my sat-nav out right now to see, it's a Sunday !)

Got a TomTom 910 (soon to be replaced with a Go 1000) and always use it as the terminal speed benchmark

Speedo accuracy according to the TT seems to be about the same on mine (speedo)until you start to do over 60mph and then you notice(even after the delay and driving as constant as you can) about 4-6mph difference (slower on TT). Not scientific but enough to give you some indication at least .

Got a TomTom 910 (soon to be replaced with a Go 1000) and always use it as the terminal speed benchmark

Speedo accuracy according to the TT seems to be about the same on mine (speedo)until you start to do over 60mph and then you notice(even after the delay and driving as constant as you can) about 4-6mph difference (slower on TT). Not scientific but enough to give you some indication at least .

Hello,

I understand that it legal for a car speedo to be up to 10% fast. That is the speedo reads faster than the real speed. Don't forget a worn tyre is quite a bit smaller than a new one, just put the spare next to a worn tyre, so for the same real speed will show a higher speedo reading.

Sat nav speeds are said to be accurate. On the motorways there are 100 metre posts at very accurate intervals so you could always use these to check speed with a stopwatch.

Regards

John Fisher

Sat nav speeds are said to be accurate.

They are BUT they're only accurate to the speed you WERE doing a few metres back and even then there's a factor of GPS signal delay involved. This also assumes that you're going in a straight line on a flat road as opposed to up an incline for example.

This topic was much discussed to great lengths at my local IAM group not so long back. We came to the conclusion that a satnav reading would be nigh on impossible to prove in terms of accuracy, calibration and speed reading at the time of the 'offence' if you were taken to a court of law for speeding, therefore you should be using the speedo of the vehicle.

All vehicle speedometers MUST over-read by law and this can be anywhere between 3 and 10% typically - it's illegal for a speedo to under-read your actual speed. There are many reasons for this.

If you were stopped by the police for speeding and you believe your speedo to be under-reading, you can ask for a calibration check as part of an appeal. If the case ever reaches court though, only 2 sources of evidence are permissible and these are the calibration check of your speedo and the reading of the policeman's speedo / speed gun (assuming it's proven to be calibrated.)

Bottom line is, only ever use your satnav speed reading as a guide and never to confirm your actual speed!

Edited 20 February, 201115 yr by scottpcs

My reason for the question was to see how close the speedo was to the real thing, for the time I slip into winter wheel mode.

Smaller wheels with altered tyre ratios etc. could mean the speedo being way slow or way fast, but you need the benchmark of the standard wheel calbration.

Scott, is that a IAM car or bike group ?

Mine was only showing a consistent difference of 3-4 mph today.

GPS is accurate, however the information you get out of your GPS receiver may not be depending upon the complexity of the receiver and the information it can add to the data that it collects from the GPS satellites.

As the speed increases so does the error.

So at 30 mph the error is one or two mph

At 70mph - tested on a long straight section of road - the error was 3 or 4 mph.

So travelling at 74 mph you are only do 70 mph -

John

PS - this is for my new vRS , the old one had the same values.

Edited 20 February, 201115 yr by jcm

This topic was much discussed to great lengths at my local IAM group not so long back. We came to the conclusion that a satnav reading would be nigh on impossible to prove in terms of accuracy, calibration and speed reading at the time of the 'offence' if you were taken to a court of law for speeding, therefore you should be using the speedo of the vehicle

Spot on

Bottom line is, only ever use your satnav speed reading as a guide and never to confirm your actual speed!

And again

It's amazing how many lorry drivers also attempt to use their tacho charts to show they weren't speeding :|

Sorry for the off topic but I thought that tacho charts where based on the speedo of the vehical?

My reason for the question was to see how close the speedo was to the real thing, for the time I slip into winter wheel mode.

Smaller wheels with altered tyre ratios etc. could mean the speedo being way slow or way fast, but you need the benchmark of the standard wheel calbration.

Scott, is that a IAM car or bike group ?

It's my IAM bike group - WYAMs (West Yorkshire Advanced Motorcyclists)

The discussion was prompted by the decision of a local examiner to fail his candidate for using his satnav as a speedometer. This would mean that his indicated speed according to his motorcycle was consistently reading over the speed limit and therefore was failed for speeding.

Personally I think satnavs should be banned from all tests and I'd even go so far as to make it illegal for satnavs to give a speed reading but of course that would be impractical and near impossible to police.

It's a common mis-conception that satnavs are more accurate than vehicle speedometers 'end of' but it's not as straight-cut as all that. Measure the indicated speed of a properly calibrated vehicle speedometer against your run-of-the-mill TomTom and there will still be inconsistencies. Just my tuppence worth.

Well that was a daft thing to do then Scott.

I tell all my associates to always use their bike speedo on test and they won't be speeding (if they remember to look at the thing) We've had two fail last year in Notts, both speeding on test as they missed two well known hidden 30's coming from national's

One told me last group night he has since passed, 2nd attempt.

My main use of the Sat-nav is storing routes via T.Y.R.E. for taking out IAM associates, means I can give really clear radio directions in good time of approaching left/right turns. It's a tool to be used, not relied 100% upon.

Well that was a daft thing to do then Scott.

I might hasten to add that it wasn't me who failed his test but I agree, it was a daft thing to do!

I only ever use my satnav as route guidance, never for speed measurement and I'd tell all my associates to do the same. Certainly I'd strongly advise against even taking the satnav on test with them, if only to remove the possibility of the examiner blaming it for excessive speed even if they don't use it for that purpose.

It's the first time we've heard of one of our members failing for use of the satnav where speed is concerned but after our discussion I'd be highly surprised if any candidates from our group still take a satnav on test with them.

Fortunately we have a good record of associates passing their test within our group but those that don't rarely fail on speed.

means I can give really clear radio directions in good time of approaching left/right turns.

You have radios within your group? We must be the paupers of the IAM world then as none of our examiners or observers in our area or group can afford them! They can be beneficial as it removes the navigation element from the test but I suppose you could argue that it can also remove some observation skills if you know what route you'll be taking on the road ahead.

Edited 21 February, 201115 yr by scottpcs

IAM PM sent Scott

I'm rather hoping my little Fabia has close to a 10% optimistic speedo ! On a regular trip home down a dual carriageway.. 50MPH limit.. I failed to notice a mobile scamera van parked on a bridge..not one I had ever seen one on before ! There were 4/5 cars in front of me (I was in the inside lane) all in the outside lane doing a similar speed overtaking fresh air..hopefully the van will have picked one of them off rather than me :p Can the scameras pick up multiple targets quickly ??

PS.. for the OP .. I have a Garmin 1310,I have no idea how accurate it is at true speed readings but at an indicated 70MPH the Garmin shows 65/66MPH Though from what I have read the speedo accuracy can vary at different speeds (?)

Edited 28 February, 201115 yr by starskybell