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Went past speed camera doing 32-33 in 30mph zone...

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Original post by M1F2R3
Oh dear, you obviously do not understand that car speedos are inaccurate, police allow 10% plus 2 mph. On the motorway I always travel over 70 MPH, why don't I get pulled over? You really should read ask a police officer rather than rant on about tyre sizes.


They do not, they allow 10% (less on bigger roads) before they pull you over. Theoretically you can travel above the speed limit for 1/4 of a mile during an overtaking manoeuvre however anyone exceeding the speed limit to any real degree will be pulled over and talked to. Even on a motorway, 75mph will get you pulled over and maybe a ticket depending on the officers mood and whether or not they've hit targets. Similarly motorway cameras are calibrated to 75 as a maximum (some are lower) and the ticket is evaluated when it spews onto the department desk. The they allow +2mph thing is a bizarre myth, there simply aren't enough traffic officers to pull over everyone so lots of people get away with speeding then justify it as they have never been pulled over. I have been told this by a close family member who was a police officer so I think its fairly accurate.

To answer the OP, probably got away with it. Some roads are confusing so its not a terrible crime but no sign, either 30 or national speed limit (these signs have considerable gap inbetween). Use your common sense.
Original post by pistachio_nut
x


No, he's exactly right, you have no idea what you're on about. Speedometers are deliberately set to show about 10% higher than your actual speed.
(edited 8 years ago)
Original post by GonvilleBromhead
They do not, they allow 10%


That's the most important word right ther. They allow for 10 but that's simply guidelines therefor not mandatory.
Original post by Gaiaphage
No, he's exactly right, you have no idea what you're on about. Speedometers are deliberately set to show about 10% higher than your actual speed.


Please provide a citation or law requiring this to be the case on all cars, as I know of some which are set exactly.
Reply 84
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10%+2 I think
Reply 85
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Original post by pistachio_nut
Please provide a citation or law requiring this to be the case on all cars, as I know of some which are set exactly.


He's actually right, at least for the UK - might be different in other countries.
Original post by pistachio_nut
Please provide a citation or law requiring this to be the case on all cars, as I know of some which are set exactly.


It's not a law, but get a SatNav and check the difference between their calculated speed and your speedo's stated speed - it's pretty much always around 10%
There's a lot of confusion about the accuracy of speedometers, but basically there are different approvals that speedos can meet. All the approvals say that the speedo cannot under-read (i.e. if it says you're doing 50, you can't be doing more than 50). They can over-read by up to 10% plus either 6mph or 4kmh, depending on the approval, for a specified range of speeds. For example at 40mph you could be indicating anywhere between 40 and 50mph, but the error isn't fixed and will change depending on your speed and other factors. A friend of mine had a car which alternated between under reading and over reading depending on how much tread was left on the tyres.

As for GPS, it can be highly accurate if you're travelling at a steady speed, but it's not always that accurate. There are plenty of errors which can creep in there as well.
(edited 8 years ago)
Original post by Gaiaphage
It's not a law, but get a SatNav and check the difference between their calculated speed and your speedo's stated speed - it's pretty much always around 10%


My car does not have this, it shows the true speed.
A satnav device would not be accurate enough at 30mph anyway.
Original post by pistachio_nut
My car does not have this, it shows the true speed.
A satnav device would not be accurate enough at 30mph anyway.


What car do you have then?
Original post by Gaiaphage
What car do you have then?



Austin Healey Sprite and a TVR Tuscan
Original post by pistachio_nut
Austin Healey Sprite and a TVR Tuscan


Right well the Austin Healey is so ancient that it won't have the vast majority of features that are now the industry standard so we can exclude that as an anomaly. The TVR Tuscan doesn't even have traction control or anti-lock brakes so again you've found an anomaly. All 'normal' cars have speedometers set to 10% faster than the actual speed, particularly because changing the tires or wearing them down due to normal driving will affect the accuracy.
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Original post by pistachio_nut
Austin Healey Sprite and a TVR Tuscan


Wow! I have to say well done for keeping them going. From what I've heard they're not exactly reliable. In fact I don't think I've ever seen any on the road.

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Original post by Gaiaphage
Right well the Austin Healey is so ancient that it won't have the vast majority of features that are now the industry standard so we can exclude that as an anomaly. The TVR Tuscan doesn't even have traction control or anti-lock brakes so again you've found an anomaly. All 'normal' cars have speedometers set to 10% faster than the actual speed, particularly because changing the tires or wearing them down due to normal driving will affect the accuracy.


Did you read the post where I explained the accuracy of speedos? Your statement in bold is far from universal. If you are travelling at x miles per hour, your speedo will indicate somewhere between x and 1.1x+6mph (or 100% and 110%+6mph, if you prefer)
Original post by Gaiaphage
No, he's exactly right, you have no idea what you're on about. Speedometers are deliberately set to show about 10% higher than your actual speed.


Original post by pistachio_nut
Please provide a citation or law requiring this to be the case on all cars, as I know of some which are set exactly.


Original post by Andy98
He's actually right, at least for the UK - might be different in other countries.


Pistachio nut is correct.

My car's speedometer also reads the true speed.

Every speedometer is different. Some speedometers read exactly true and some speedometers over-read. If the speedometer overreads it can do so by any amout it is set to. Some are set to overread by a couple of percent, others 5%, 10% or even more. Legally speedometers can overread by up to 10% plus a few mph on top - quite a significant extent.

In reality most modern speedometers overread by some amount but less than 10%. Some by up to or over 10%. Some like mine and pistachio's are entirely accurate - this is normal and legal (and quite convenient) and to insist otherwise is silly.
Original post by DeeWave
Pistachio nut is correct.

My car's speedometer also reads the true speed.

Every speedometer is different. Some speedometers read exactly true and some speedometers over-read. If the speedometer overreads it can do so by any amout it is set to. Some are set to overread by a couple of percent, others 5%, 10% or even more. Legally speedometers can overread by up to 10% plus a few mph on top - quite a significant extent.

In reality most modern speedometers overread by some amount but less than 10%. Some by up to or over 10%. Some like mine and pistachio's are entirely accurate - this is normal and legal (and quite convenient) and to insist otherwise is silly.


I'm sorry but this is just not true.

The Motor Vehicles (Approval) Regulations 2001 states that "the speedometer must never show an indicated speed less than the actual speed". I don't know if you are aware of how a speedometer works, but it uses the rotation of the axle and the estimated tire circumference to calculate your speed. If your tyres are very well pumped up, or brand new so have a very large tread, then the circumference will be greater than expected and so your car will travel faster than the speedometer thinks it is. If you were to then get caught speeding, you could easily sue the manufacturer and they quite clearly don't want that!

There are therefore two reasons why speedometers show a higher speed than you're actually going:

1. Even if the speedometer is calibrated so at the maximum pressure and tread it is 100% accurate, any less than maximum pressure and any reduced tread due to natural wearing down will give a reduced circumference so you will go slower than your speedometer says.

2. Speedometer manufacturers always over-compensate to ensure that they cannot be sued by speeding drivers.

So unfortunately, unless your speedometers work by a different method or you get new tyres every mile, they do not show the correct speed.
32mph is fine... 33 IS SERIOUSLY RISKY TERRITORY, LIKE BEING ON A TIGHT ROPE.
Original post by Gaiaphage
I'm sorry but this is just not true.

The Motor Vehicles (Approval) Regulations 2001 states that "the speedometer must never show an indicated speed less than the actual speed". I don't know if you are aware of how a speedometer works, but it uses the rotation of the axle and the estimated tire circumference to calculate your speed. If your tyres are very well pumped up, or brand new so have a very large tread, then the circumference will be greater than expected and so your car will travel faster than the speedometer thinks it is. If you were to then get caught speeding, you could easily sue the manufacturer and they quite clearly don't want that!

There are therefore two reasons why speedometers show a higher speed than you're actually going:

1. Even if the speedometer is calibrated so at the maximum pressure and tread it is 100% accurate, any less than maximum pressure and any reduced tread due to natural wearing down will give a reduced circumference so you will go slower than your speedometer says.

2. Speedometer manufacturers always over-compensate to ensure that they cannot be sued by speeding drivers.

So unfortunately, unless your speedometers work by a different method or you get new tyres every mile, they do not show the correct speed.



If we're splitting hairs then no speedometer is entirely accurate. In answer to your question yes I do know how speedometers work.

Whilst it's true that tyre wear will lead to the speedometer overreading, in terms of the general accuracy of speedometers this is negligable. Wearing a tyre from full tread to no tread will make just over 1% difference to the circumference which works out at around 1mph or there abouts at 60mph. Speedometers only need recalibrating if the wheel size is changed.

It is patently untrue that all speedometers overread so you shouldn't continue to insist it. At least two contributors to this thread own cars where the speedometer is dead on to within the natural reading tolerance of a speedometer = +/- 1mph. Perhaps if I run all my tyres bald then my speedometer will begin to overread by 1mph at motorway speeds.

I can say with certainty my speedometer reads with less than 1mph inaccuracy which is as accurate as a normal analogue gauge can be due to reading error. Therefore my speedometer is reading true and is NOT overreading thus proving that speedometers can be accurate.

You would be right if you said that MOST speedometers are programmed to overread but you are NOT correct in saying they ALL are.
Please use English,I don't understand your post.
Reply 99
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Original post by Johnny G.
Please use English,I don't understand your post.


This thread is well over a year old.....

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