No, it's for their equipment (and the way they use it). Car speedometers are governed by regulation, with very substantial penalties and recalls for car manufacturers who are found to be out of compliance! For mainstream cars, the speedo type approval requirement is that it can OVER-read up to 10% +2.5 MPH, but it cannot UNDER-read - under any circumstances. For this reason, manufacturers always set them a bit high. Check pretty much any mass-produced car's speedo against a GPS and it will be 2 or 3 MPH higher than true speed, when fitted with the correct tyres. In reality, this means that prosecutions for speeding in a 30 zone, where the car's speedometer was reading less than about 37, are pretty rare.
How to check if you have been caught in a police speed trap
- Thread starter oldcardriver
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No, it's for their equipment (and the way they use it). Car speedometers are governed by regulation, with very substantial penalties and recalls for car manufacturers who are found to be out of compliance! For mainstream cars, the speedo type approval requirement is that it can OVER-read up to 10% +2.5 MPH, but it cannot UNDER-read - under any circumstances. For this reason, manufacturers always set them a bit high. Check pretty much any mass-produced car's speedo against a GPS and it will be 2 or 3 MPH higher than true speed, when fitted with the correct tyres. In reality, this means that prosecutions for speeding in a 30 zone, where the car's speedometer was reading less than about 37, are pretty rare.
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The rolling radius of a tyre, varies very little between new and worn out.
Very simplistically and ignoring other factors; based purely on tread depth, assuming a start depth of 9mm and finish depth of 1.6mm (going through the numbers for my tyres), the number of tyre rotations over a mile varies by 3.4%.
Edit. Sorry, that should read - the distance travelled by the variation in rotations equates to 3.4%
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Yes, fair enough.
And as I said earlier....
I think my point was, that given the inaccuracies of our speedos, we all know they don't read the true speed.
If there was going to be zero tolerance for breaking the speed limit, I would wish that my speedo was properly calibrated.
It's technically possible to do, but likely to be prohibitively expensive.
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These days, the over-read is done in software. My conventional moving pointer speedo over-reads by the programmed in 10%, call up, on the digital display the speed, and it shows the true speed, which always agrees with my dash GPS reported speed. For emergency services use, the software reports true speed.
One company car I had, showed the true speed on the dial speedo, my guess was that it had been accidentally setup on the production line, for the emergency services use.
I find that whenever I go past a SID, (speed indicator device), my speedo will say 32mph but SID always says I'm doing 29mph.
On a 40mph stretch it will read as 39mph while my speedo says 43mph On a recent long journey to Dorset my speedo would be at 60mph, (I was towing a small trailer), and both my Satnav and a SID indicated I was doing 56mph.
So, to play safe I always try to drive so that my speedo says the correct MPH for the area, meaning that I will actually be doing slightly less than the official speed limit.
On a 40mph stretch it will read as 39mph while my speedo says 43mph On a recent long journey to Dorset my speedo would be at 60mph, (I was towing a small trailer), and both my Satnav and a SID indicated I was doing 56mph.
So, to play safe I always try to drive so that my speedo says the correct MPH for the area, meaning that I will actually be doing slightly less than the official speed limit.
Very unusual to have the car's digital display disagreeing with its pointer?! I've not seen that before.
A 3.4% error on an analogue speedo is not noticeable.
Yes, possible, but the errors are usually not linear. At higher speeds you get a bit of tyre "growth" - plus, of course, as you said earlier, a small percentage for wear - maybe call it 5% when adding different loads and inflation pressures into the mix. The easiest thing, of course, would be to simply hack a bit of software to tell the speedometer to display whatever the GPS is telling it, and forget about tyres, but manufacturers can't do that, because under the type approval regulations, they have to take "ownership" (and thus all liability) for any inaccuracy at the point the car leaves the factory. No homologation engineer in his right mind, would sign anything like that off, knowing that part of his "speedometer system" was under the control of the American military (or whichever GPS network he's using)!
Maybe because the digital display is not a standard user feature, it needed the ECU reprogrammed, to add the feature. It is now on the button, which selects what the dash display shows - fuel range, average speed, miles per gallon, fuel left in each side of the tank and true speed. They all use true speed, in their calcs - rather than speed + 10%
Agree, even at 30 it will read 29 or 31 instead, that's easy to spot on a speedometer. Harry should go to Specsavers.Yes it is! At the "ton" it would be saying 103.4!
Sorry, I must disagree...+1 or -1 on an analogue speedo, when you are driving, is simply not noticeable. I think I am correct in suggesting that many car speedos do not even have the 1mph division marks..
And, - As it happens, I went there yesterday afternoon for a checkup - absolutely no change at all, just as good as they always were - and surprise, surprise, they didn't try to upsell me with an expensive pair of reading spec., they said my usual poundshop ones were perfectly adequate. So that's no change in prescription, for 25 years..
You can't see if the needle lines up with the 30 mark on your speedo?
Ah... That makes sense!
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