If you set the cruse control to, say 70mph and put the trip computer onto average speed then press the trip reset button. The desplay will say "---mph" for a few moments then come up with a number. This will be the true speed, as measured as accuratly as the tyres and sensors allow.
Assuming your in a 70 limit, increase your speed and test again, once the "---mph" changes to "70mph" and say your speedo says 77mph, then you know the error of your speedo.
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There are set standards on this.
The Road Vehicles (Construction and Use) Regulations 1986, as amended, 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 than 10 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.
So, for production cars, if you are physically moving at 70mph, your speedo cannot read less than this but can read 79.7mph.
Most manufacturers work to the european limit of 10% + 4km/h.
So 70mph true speed could read 79.5mph on the speedo. This is acceptable as in the range 25mph to 70mph it still is covered by the british requirements.
If you take it a stage further:
Most patrol car cops wont bother about anybody unless your doing at least 10% + 2mph over the limit. Generally they set the speed cameras anywhere between 10% or 10% + 5mph.
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I know (as i asked somebody) that the speed cameras on the Dumfries and Galloway section of the A74(M) motorway are set at 80mph real physical speed.
The same road in Strathclyde is set to 78mph real physical speed.
So, if your speedo is reading worst case within the standards (which its probably not!) (then its saying 79.5mph for doing 70mph). The 80MPH camera will trip at 90.5MPH speedo reading. The camera doesnt think your doing 90MPH, it thinks your doing 80.
Now, to allow of tyre diamater changes and other real margins of error, most car manufacturers opt for a 5% + 4km/h over approach so your looking at needing 86.5MPH speedo reading before the camera trips or patrol car says stop. Again, the patrol cars laser thing says your doing 80 and youll probably not get stopped, unless its a quiet day or there are dangerous conditions.
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with regards to the service thing, your car odo will read correct miles.
Meaning, if you travel for 1 hour at 70mph on the speedo, you wont have registered 70 miles.
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On satnav's, they are right to about 1mph if your going level and they have a good view of the sky and your not turning a corner.
Make sure you have the logging function on, as if you do get a ticket, the satnav log is admissible to prove what speed you were doing.
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Hope this fills some things in for you guys.
ps. im not responsible if you get a ticket or break any laws whilst trying to find what speed cameras and patrol cars will catch you!
