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[MutantCav]

My satnav says the car is travelling at a slightly different speed to the speedo...if the satnav is correct then the the speedo is overreading by about 5%.

Is the satnav likely to be the more accurate?? If so will having bigger or smaller wheels make the speedo more accurate and what size would roughly speaking do the job??

[Lazydocker]

Sat Nav is affected by the curvature of the earth etc... Speedo should over read by about 3 mph IIRCC

My Sat Nav confirms this is normally the case when compared to the GPS speed

[MutantCav]

Yep probably about right...speedo is about 3-4mph over the satnav...

[Hannah Judes Dad]

When I had an Audi RS6 the satnav and speedo both showed the same speed on the motorway (that was before I got it de-restricted) so I can say I know what 155 mph is but 180 in the same car,not so sure.

[Kevin Wood]

Speedos are always calibrated to read slightly high.

A satnav, assuming it is displaying GPS speed, not speed interpolated between points on the map, will be pretty accurate in certain conditions, namely with a clear view of the sky (no tree cover), plenty (more than 4) strong satellite signals and on a straight course.

Errors can be as low as 0.1 MPH from a decent GPS receiver but you can't really rely on it because it won't be as accurate in all conditions.

Kevin

[TheBoy]

Portable car based satnav isn't particularly accurate for speed.  The speedo generally is set to read approx 2mph over, assuming correct tyres

[TheBoy]

Speedos are always calibrated to read slightly high.

A satnav, assuming it is displaying GPS speed, not speed interpolated between points on the map, will be pretty accurate in certain conditions, namely with a clear view of the sky (no tree cover), plenty (more than 4) strong satellite signals and on a straight course.

Errors can be as low as 0.1 MPH from a decent GPS receiver but you can't really rely on it because it won't be as accurate in all conditions.

Kevin

Thats the key

[feeutfo]

all speedos over read for legal reasons, unless calibrated, also for legal reasons.
 I would personally trust a decent gps signal over any uncalibrated speedometer, rightly or wrongly.

To my thinking, again rightly or wrongly, if you sit at a speedo indicated 50mph in an averge speed trap,  between 2 cameras for inatance, and check your gps you may well find 47 showing depending on the speedo error. That gives an indicated 53mph as still legal, add a couple of mph leeway on the camera and i dont worry too much about doing 55 in that situation.

In my experience usuak over read is between 7 and 10%.

Re tyre size i would guess a slight increase in outside diameter? a wheel size calculater might help.

[I_want_an_Omega]

Thats what the number is that comes up wehn you press both buttons on the trip computer stalk @ the same time.

IIRC it needs a TECH2 to change it.

Tyres being underinflated with give an incorrect reading.

[dbr]

My satnav says the car is travelling at a slightly different speed to the speedo...if the satnav is correct then the the speedo is overreading by about 5%.

Is the satnav likely to be the more accurate?? If so will having bigger or smaller wheels make the speedo more accurate and what size would roughly speaking do the job??

Would trust sat nav to be more accurate. GPS initially has come from military souces..My speedometer is about 2/3 mph over at about 30mph on sat nav and the radar operated signs. The error increases with speed to about 6mph at 70. i.e 76 on speedometer is 70. You could try sitting at 60mph and timing between mileposts. It should be one minute.

[TheBoy]

My satnav says the car is travelling at a slightly different speed to the speedo...if the satnav is correct then the the speedo is overreading by about 5%.

Is the satnav likely to be the more accurate?? If so will having bigger or smaller wheels make the speedo more accurate and what size would roughly speaking do the job??

Would trust sat nav to be more accurate. GPS initially has come from military souces..My speedometer is about 2/3 mph over at about 30mph on sat nav and the radar operated signs. The error increases with speed to about 6mph at 70. i.e 76 on speedometer is 70. You could try sitting at 60mph and timing between mileposts. It should be one minute.

GPS has the potential to be more accurate with excellent signals.

Satnav and GPS are not the same thing .

[Entwood]

GPS does NOT ever actually give any indication of speed. All it does, given the reception of 4 satellites is to extremely accurately give a 3 dimensional plot of your position.. in simple terms... latitude, longitude, height.  Nothing else at all.

The software then takes over, and, for example in an aircraft, will calculate 2 positions 1/5 second apart and so work out a speed. The shuttle space craft, I'm told, does it every 1/100 of a second !!

This is used as a backup to a ring laser gyro with built in accelerometers, to give an accurate read out of position height and speed. And the kit costs lots !!!

Your satnav uses the same technology to derive its position ... but the rate/number of calculations is far, far smaller. Therefore the time between the calculations is much higher, so the sped accuracy is far less.

If you travel at a constant speed on a straight and level road for at least 30 seconds, it is probably quite good. As soon as your speed changes, your height changes, or the direction changes, then the satnav makes various assumptions which all lead to inaccuracy.

This can easily be seen, travel at 15 miles an hour, brake very hard to a halt and watch the speed readout ... it will still be decreasing whilst you are stationary ... only a moment .. but it will ... that is the "averaging" effect of cheap computing of GPS.

[dbr]

My satnav says the car is travelling at a slightly different speed to the speedo...if the satnav is correct then the the speedo is overreading by about 5%.

Is the satnav likely to be the more accurate?? If so will having bigger or smaller wheels make the speedo more accurate and what size would roughly speaking do the job??

Would trust sat nav to be more accurate. GPS initially has come from military souces..My speedometer is about 2/3 mph over at about 30mph on sat nav and the radar operated signs. The error increases with speed to about 6mph at 70. i.e 76 on speedometer is 70. You could try sitting at 60mph and timing between mileposts. It should be one minute.

GPS has the potential to be more accurate with excellent signals.

Satnav and GPS are not the same thing .

The technology, i.e signals from at least 3 satellites helps your sat nav determine where you are. The nav part surely depends on the maps installed. I may be wrong but thats the basic concept and its origins

[dbr]

GPS does NOT ever actually give any indication of speed. All it does, given the reception of 4 satellites is to extremely accurately give a 3 dimensional plot of your position.. in simple terms... latitude, longitude, height.  Nothing else at all.

The software then takes over, and, for example in an aircraft, will calculate 2 positions 1/5 second apart and so work out a speed. The shuttle space craft, I'm told, does it every 1/100 of a second !!

This is used as a backup to a ring laser gyro with built in accelerometers, to give an accurate read out of position height and speed. And the kit costs lots !!!

Your satnav uses the same technology to derive its position ... but the rate/number of calculations is far, far smaller. Therefore the time between the calculations is much higher, so the sped accuracy is far less.

If you travel at a constant speed on a straight and level road for at least 30 seconds, it is probably quite good. As soon as your speed changes, your height changes, or the direction changes, then the satnav makes various assumptions which all lead to inaccuracy.

This can easily be seen, travel at 15 miles an hour, brake very hard to a halt and watch the speed readout ... it will still be decreasing whilst you are stationary ... only a moment .. but it will ... that is the "averaging" effect of cheap computing of GPS.

Those guys in the Apollo were brill. The average car now has more computing power.  Also brill how they use Gyro's to change the orbit of the space station.

[hotel21]

A car speedo is required to be accurate to within 10%.  Reaqlity is that it will over read as otherwise, the speedo could give a readout whereby it said you were doing 30 and in reality, you were doing 33 outside your local school.  Given the propensity of the Talivans, the least you could expect is a warning letter, if not a nicking, given current enforcement measures/campaigns.....   

So, speedo will over estimate your speed - you want to see the magic fast numbers anyway, as per the published figures, sooner rather than later anyway??   

As said by others, satnavs use a geopositioning system that works in 3 axes and thus is greatly affected by height.  For height, read road inclines and undulations and thus, it assumes an arrow straight path from A to B between display indications.

This introduces a margin of error which may or may not be less than the 10% speedo reading.

In reality, as far as I know, those who are subject to legal proceedings who give satnav speeds as a potential defence are in the situation where its interpretation is subject to legal presentation.

In other words, whomsoever talketh the most 'dangle berries' to influence the Judge, may win....   



or not........