Slightly OT but how accurate is your car speedo?

[+Alboy]

Assuming that my GPSr is more accurate technology than my car's speedo, I drove at a constant 50mph according to the car, but my GPSr only showed 46.5mph. I'm happy it errs on the side of caution, by thought it might have been more accurate.

 

Have you checked and is your cars speedo more accurate?

[+chizu]

Just remember your car will measure the speed in a straight line from A to B including altitude. Your GPSr will not include the altitude change and therefore measure less distance travelled and so estimate a slower speed than you're actually doing.

[+rutson]

Markwell...

 

Markwell...

[nobby.nobbs]

so your defence when in court will be you know that you weren't speeding because you were staring at/ playing with a gps while driving....... you don't want a licence for long then?

[+Carlos & The Birdie Crew]

I've been checking my GPSr against my speedo for ages now and I've just had more proof that the GPS is the more accurate.

A new bypass has been built near me and a speed warning device added to deter speeding as you approach the built up area and 30 mph limit. The difference between this device and others I've seen is that it gives you your sppeed in mph as you approach it (as long as theres not too many other cars before or after you). I've set my cruise control at 30, 35, 40 and 45 (by the GPS) as approach this and, guess what, it's bang on in every instance.

A reason for the discrepencies, I think, is the differing tyre sizes available on various models of car. The final rolling radius of a tyre is key to the speedo's accuracy. Even an over or under inflated tyre can change the readings a little.

For info my speed is on average 10% out at all the key speeds I've tried and I believe that the courts must allow for speedo inaccuracy of 10% also. So it could be said that diving at 76mph is within the law but your speedo could be reading nearly 84 mph. Would you want to risk it? Not me ;-)

Carlos

[+chsims1]

Car manufacturers have a legal responsibility to fit a speedometer which doesn't show a slower speed than you are actually doing. It should be no surprise that generally they use units which indicate a +10 % reading.

[+Papakas]

I've been using my GPS in car for more years than I can remember! Originally bought a GPS for motorcycle rallying and over the years have measured with intrigue speedo differences. Generally I've found the car speedos I've assessed as more accurate than motorcycles. I've found the difference is normally up to 10% that's 63 mph GPS at 70 mph speedo, however I've generally seen an average of -5 mph at 60/70 mph.

 

Best I've seen is a Land Rover Defender 90 I hired from Manchester for a 2000 mile business trip in Scotland, I was seeing -1.5 mph at 60 mph! I once owned a '89 reg Volvo 760GLE returning -2 mph at 70 mph and have recently measured my Directors Merc 320CLS (in black so we've nicknamed it Batmobile!) at the same.

 

As for my current crop of travel bugs, P99GPS -4.5 at 60 mph (Garmin iQue3600), P100GPS -5 at 70 mph (Garmin Quest2) and P111GPS -4 at 70 mph (Garmin iQue3600).

[+t8r]

nissan is being sued over a related issue

[Red Squadron]

Assuming that my GPSr is more accurate technology than my car's speedo, I drove at a constant 50mph according to the car, but my GPSr only showed 46.5mph. I'm happy it errs on the side of caution, by thought it might have been more accurate.

 

Have you checked and is your cars speedo more accurate?

 

Mine too, you are not in the minority. My speedo seems to be about 3 or 4 mph slower than the GPS (TomTom 510) readout.

[+Jonovich]

Even an over or under inflated tyre can change the readings a little.

 

Can it? The circumference of the tyre remains the same due to the construction of the tyre, so 1 revolution equals the same distance. As a tyre wears I can see the speedo accuracy being altered slightly, more so on a mud terrain 4x4 tyre where the tread is much deeper and therefore there is a larger difference between a new an worn tyres circumference.

 

J

[+Simply Paul]

I was in an Axiam which, with its twin cylinder diesel engine at full chat was doing an indicated 55mph. We went past one of those 'mind your speed' signs and it flashed up that we were doing 42 in a 40 zone. Naughty naughty.

[+Carlos & The Birdie Crew]

Even an over or under inflated tyre can change the readings a little.

 

Can it? The circumference of the tyre remains the same due to the construction of the tyre, so 1 revolution equals the same distance. As a tyre wears I can see the speedo accuracy being altered slightly, more so on a mud terrain 4x4 tyre where the tread is much deeper and therefore there is a larger difference between a new an worn tyres circumference.

 

J

 

Well on more modern tyres (run flats) maybe that is true but if the distance between the axle centre and the road surface (radius) is lessened (under inflated) then the circumference is also reduced (the footprint of tread on the floor is larger with less pressure) and a rolling circumfrence should be taken with the weight of the car on it, try it. You'll need a pump to blow your tyre back uop again though

I've been through the mathmatical trauma of this exact argument when involved in rallying as the "power at the wheels" was affected by pressure or tyre type used (slicks or knobblies) when setting our car up on a rolling road.

 

Cheers Carlos

[+Jonovich]

Well on more modern tyres (run flats) maybe that is true but if the distance between the axle centre and the road surface (radius) is lessened (under inflated) then the circumference is also reduced (the footprint of tread on the floor is larger with less pressure) and a rolling circumfrence should be taken with the weight of the car on it, try it. You'll need a pump to blow your tyre back uop again though

 

Hmm... I still can't believe it makes a difference to speed. The circumference of the tyre does not change at all as it is pretty much rigid. Balloons increase their circumference the more you inflate them, tyres do not, the steel in the multi ply construction prevents this. Therefore unless the tyre is slipping on the rim, one revoultion of the wheel hub equals the same amout of distance travelled. The distance between the axle centre and the ground is only an issue if the shape of the tyre remains circular, and flat tyres are anything but circular!

 

J

[+Colourbox]

The speedometer is an indicator of speed, they are usually not accurate normally displaying up to 10% difference from the actual (true) speed, this measurement is set in the positive plain therefore you should not be speeding if your needle is on the 70 MPH mark on a motorway, and will likely be driving at ~ 65 MPH

 

There are good reasons why they are not accurate but they are many and varied, from changes in tyre size, pressure and worn out transmission. Car manufacturers, do not want you pointing your finger at them in the any prosecution offences that you might be involved in.

 

Pippa

[+Carlos & The Birdie Crew]

Dakar - I may have to concede to your argument but not till I've confered with other rally pals over many beers this evening. Cheers!!

[+Jonovich]

Dakar - I may have to concede to your argument but not till I've confered with other rally pals over many beers this evening. Cheers!!

 

I've been thinking more about this and in extreme cases when the tyre is severely over inflated then the tread will 'bulge' and increase the circumference and in that case I also have to concede that the speed of the vehicle will increase without any change to the indicated speedo reading.

 

We have of course been talking specifically about speed here, incorrectly inflated tyres will seriously affect performance, efficiency and prevent you getting the optimum from your vehicle, so for rolling road tests, getting it right does matter.

 

Jon