Indicated Ground Speed and distance covered?

[viscount]

When I purchased a Garmin Sat Nav, I noticed a difference between the speed shown on the speedometer on the dash-board and the speed displayed on the Sat Nav. Being an older car I simply took it to be a factor of age. At 73 mph indicated, the Sat Nav would show 70 mph - the error being constant on every trip.

I've been told that the Sat Nav speed is more accurate than that displayed by the car speedometer. Is that indeed the case, anyone confirm?

I've recently changed my car, and now have a very much newer one. To my concern I find that the difference is greater. 75 mph indicated, shows at 70 mph Sat Nav. As I'm not going to risk driving at 100 mph to observe the percentage error without using mathematics, I guesstimate the error to be around 7%.

Where I am getting to in this ramble. Well, the conspiracy theorists could accuse the motor industry of trying to slow traffic down by providing inaccurate information to the driver. My concern is that if the speedo is under-reading, then the total distance covered by the vehicle over it's lifetime is under-stated too. In nice round terms, if I sell my car with 100,000 miles 'on the clock', the vehicle will actually have travelled 107,000 miles, maybe not that significant, but would it be a knowingly fraudulent statement?

As a matter of interest is Police car-based speed recording taken from the speedo, or from Sat Nav information?

[Deleted]

Some years ago I worked with a senior engineer from Rolls-Royce in Crewe (when they were still building the cars there). This is what he told me when we got into this discussion one day.

He said that odometers (as he called them, being an engineer , are notoriously inaccurate, even on RRs. Apparently this is due to the connection between the road wheels and the odometer, which is rudimentary to say the least. This is simply down to cost and there is an unwritten rule amongst motor manufacturers (and presumably the regulatory authorities) that so long as they are within 10% of the actual figure, it was OK. Bizarrely (and he couldn't explain why) lorries' speed and mileage measurements are more accurate, This is probably to do with the tachograph arrangements.
My next question was - "How does that work with the police then?" His answer was that the police are aware of this and will not normally prosecute if people are 10% over the limit, because if you get a car-savvy solicitor he will use this knowledge to get people off.

With respect to GPS, I believe you are right in thinking that it is more accurate. With my aviation hat on, compare the TAS (True Air Speed) against what the satellites are telling you and you will see little or no difference on the instruments in front of you.

Afaik, the police systems are fairly rudimentary. If they're following you, they rely on the video (I don't know how accurate that is), or if it's the hairdryer or the stationary van, then it's based on time and distance, just like the speed cameras. (The RR man said they have an allowance of 10% on them too).

On a more humorous note, I worked with a guy some years ago who was doing up an old Triumph Spitfire. As part of this work, he decided to 'adjust' the speedometer. As he opened it, a small piece of paper fell out. On it were written the words 'Oh no - not again!'

Just my two penn'orth. I am sure others may have more detailed info.

[Beemer]

The van I drive at work, a Ford Transit Connect is governed by the company to 70mph and has a disc on the back saying so. My satnav shows that flat out it will only do 65 so I changed the disc to 65. In the sixties when we all had motor bikes and would return back to the cafe bragging that we had "done a ton" had probably only done about 90mph, satnavs would have took the shine off it for us as not many were capable of 100. Regards Beemer.

[GHB]

I was on a course in Wales a few years back and it was stated that a lot of drivers (especially youngsters), who customized their cars by putting bigger wheels on had been caught speeding. This is because speedometers and odometers are calibrated to that vehicles standard wheel / tyre size. Thus putting a larger size on lowered the displayed speed (eg. showed 50mph when doing 55mph) and 'clocked' a longer mile. Of course the opposite happen if you put a smaller wheel on (shows 55mph when doing 50mph) etc.

So just think on if you stray from your vehicles standard wheel/ tyre size!

Regards CliffW

does this make sense?

[Biggles]

I think you have highlighted something that needs deeper investigation, one reason is speed tickets as it would seem that the speed varition from vehicle, GPS is quite large, so where in the equation does static speed cameras come in ?. I recall many years ago with speeding offences that there was roughly a known 10% allowance for error before a ticket or summons was issued. Even despite the speed ticket thing I think it needs looking into.