Work with ex-policeman, one of the courses they did was on the various forms of speed trap, they where told that the average speed cameras are set at 58

strange how we’re so obsessed with this – seems like quite a lot of fuss and effort to save perhaps 1 minute off your journey time

Foreign lorries are not always limited are they?

Our Mondeo used to under-read at slower speeds (so 25mph actual would read around 23mph on the speedo) and then as you passed 30-40mph it would zero out and start to under-read to about 5% at 70mph.

My point was has anyone been caught, or know of anyone who has been caught.

It seems no one has and I have never heard of anyone who has, which seems very strange to me.

So your car says 50mph and the satnav 45mph? If so that’s illegal, there’s a 10% tolerance on speedos in cars but it’s only -10%. They can under read by up to 10% but never read over the true figure. The only exception to this might be if you’ve got non standard wheels on with a different diameter to the original ones which would mess the speedo up.

Other way round, car speedo can over read by up to 10%, TBH some car manufacturers seem to do this as a matter of routine, Ford & Nissan cars have hopeless speedos in my experience. I had a Nissan terrano and for that to be doing 70mph as indicated by a sat nav, it was doing an indicated 80mph. If you drove it according to the speedo there was no chance of getting caught speeding and it helped with the fuel economy as well.

deadlydarcy – Member
Going eastbound on that long 50mph stretch through Wales today, a load of cars shot past at around 70. “Eejits” I was thinking. Sure enough there was a couple of mile stretch at the end with no camera before it changed back to 70mph

did they have blue lights and make a funny noise?

What I’d like to know is how people know that their speedo is under-reading? I mean I accept that there will be an error associated with every speedo as that is the nature of all measuring devices, but what are people comparing their speedo readings to and how do they know what the accuracy of the coomparative figure is?

What I’d like to know is how people know that their speedo is under-reading? I mean I accept that there will be an error associated with every speedo as that is the nature of all measuring devices, but what are people comparing their speedo readings to and how do they know what the accuracy of the coomparative figure is?

One assumes it’s their GPS speed readings on SatNav. Anecdotally, I hear that SatNavs (given 5 or so satellites) are quite a bit more accurate than your speedo. But exactly how accurate they are, I don’t know.

My point was has anyone been caught, or know of anyone who has been caught.

It seems no one has and I have never heard of anyone who has, which seems very strange to me.

My sister’s mate on the M1 in norf London, near Watford.

Other way round, car speedo can over read by up to 10%, TBH some car manufacturers seem to do this as a matter of routine, Ford & Nissan cars have hopeless speedos in my experience. I had a Nissan terrano and for that to be doing 70mph as indicated by a sat nav, it was doing an indicated 80mph. If you drove it according to the speedo there was no chance of getting caught speeding and it helped with the fuel economy as well.

My old Focus used to read about 33/34 when it was actually doing 30, so I reasoned that with leniency in camera settings to allow for car mis-calibration (I was lead to believe this was 10%) that my doing 55 in a 50 with average cameras would be fine.

You can only calibrate a speedo for a certain tyre wear as the speedo guesses the speed on an assumption about distance travelled per wheel revolution.

On a small diameter tyre (say 14″) the tread could vary by up to 8mm (on both sides) ie 16mm diameter variation on a 352.8mm diamater ie roughly 5% variation in speed reading over the life of the tyres…

what are people comparing their speedo readings to and how do they know what the accuracy of the coomparative figure is?

The distance markers by the roadside. Little white posts with distances marked off in tenths, so if you have a breakdown you can pinpoint your location to the rescue services.

Another Q. What are car milometers calibrated from then? If my speedo is 5% over, does that mean the 60K my car has done is really 57K?

Another Q. What are car milometers calibrated from then? If my speedo is 5% over, does that mean the 60K my car has done is really 57K?

Yep. Most probably. And you’re also doing less MPG than you think too!

All I keep reading is a friend of a friend got caught etc., I usually do around 60mph in them and not been caught. Not big and not clever but still not reading about any direct experiences here…

Sat Nav’s are very accurate (less than 1m a second accuracy easily). They’re not computing your speed from your postion tho, they’re using he dopplar shift on the signal, so not having a large number of saterlites in view has less effect on accuracy.

I seem to recall the police calibrate against sat navs.

This BBC report would suggest that the S Wales ones are working fine.

wiilt – Member

My point was has anyone been caught, or know of anyone who has been caught.

Monksie off here got done by a camera in some roadworks. He ‘fessed up. There was a thread about it but I think it was before The Hack