Speedo accuracy

BombayMix

BombayMix

BombayMix

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Joined
24 May 2008
Posts
1,112
I’m wandering what the expect accuracy of your speedo. Just all those signs showing your speed are always 2-3mph more the speedo says. Around here that could be enough to get a ticket with all the speeding cameras.

Should I get it checked?
 
I'm sure those signs overestimate ridiculously! (i'm assuming you mean the scoreboard ones which show your speed as you approach?)

I've passed one doing 27-28 indicated (about 25 on sat nav) and it's told me i'm doing 32! :p
 
Those signs are about as accurate as a ****ed sniper. In most cases, speedos read a few MPH higher than you're actually doing. If you're worried, turn your sat nav on and drive at a constant speed on a straight road to compare. :)
 
BombayMix said:
I’m wandering what the expect accuracy of your speedo.
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Reminded me of Allo Allo:

Lry1x.jpg


Anyways, you speedo should over read for any given speed.
 
Legally, speedometers can read upto 10% above true road speed. They cannot read below true road speed.

The source is on the internet if you can be bothered to waive through the bureaucratic language of a goverment bill.

It's this legal requirement that created the myth/legend of police officers allowing drivers to speed by upto '10% + 2mph' before issung a ticket.
 
The "But my speedo said" defence is never going to work. A speedo will happily tell you you're going faster than you think. If you think about it this isn't a problem, if you're travelling at an indicated 40mph you KNOW that you are not travelling any faster then 40mph - so no worries about tickets etc.
The speedo is allowed to air on the side of caution - so by being out that way you'll never be travelling any faster than indicated.
If it was the other way around you could be at an indicated 40mph, but actually travelling at 44mph = not so good.
 
Bear in mind, if you've changed wheels or tyres, then this will have an impact on the accuracy of your speedo. As will simply driving the car; as your tyres wear down, the wheel will have to turn more times to cover the same distance, resulting in an over-reading.
 
Haggisman said:
I'm sure those signs overestimate ridiculously! (i'm assuming you mean the scoreboard ones which show your speed as you approach?)

I've passed one doing 27-28 indicated (about 25 on sat nav) and it's told me i'm doing 32! :p
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Yep I mean those signs. I use the sat nav on my phone but the speedometer on it hard to read. I'll download a dedicate app to check.

stoofa said:
If it was the other way around you could be at an indicated 40mph, but actually travelling at 44mph = not so good.
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Exactly what I was thinking. But basically you are all saying the speedometer should be showing my speed as faster then it actually is. Mean those boards are way off!

Oh well as long as the boards show a happy face everyone happy in the back! :D
 
Tomsk said:
Legally, speedometers can read upto 10% above true road speed. They cannot read below true road speed.

The source is on the internet if you can be bothered to waive through the bureaucratic language of a goverment bill.

It's this legal requirement that created the myth/legend of police officers allowing drivers to speed by upto '10% + 2mph' before issung a ticket.
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ubersonic said:
Really? I thought it was the official ACPO or whatever their called guidelines stating as such :p
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The ACPO guidelines set out the minimum figures at which a Fixed Penalty Notice and a Court Summons would normally be considered appropriate for a speeding offence, if there are no other aggravating circumstances. The minimum figure for a FPN is the posted limit + 10% + 2 mph. These figures are shown in the table below:

The ACPO Speed Enforcement Guidelines
Speed Limit (mph) Fixed Penalty Summons
20 25 35
30 35 50
40 46 66
50 57 76
60 68 86
70 79 96


The full details can be found on the ACPO website.

Bear in mind that exceeding a speed limit by any amount whatsoever is an absolute offence, and a police officer is fully entitled to charge a driver for doing, say, 31 in a 30 limit outside a school at closing time, or in a busy High Street.
 
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Greebo said:
The minimum figure for a FPN is the posted limit + 10% + 2 mph.

Bear in mind that exceeding a speed limit by any amount whatsoever is an absolute offence, and a police officer is fully entitled to charge a driver for doing, say, 31 in a 30 limit outside a school at closing time, or in a busy High Street.
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Those 2 sentences seem to be in contradiction with each other?

If the minimum for an FPN in a 30 is 35, then what punishment would be issued below that?
 
Haggisman said:
Those 2 sentences seem to be in contradiction with each other?

If the minimum for an FPN in a 30 is 35, then what punishment would be issued below that?
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From a speed camera, nothing (if ACPO guidelines are being followed by the local camera partnership). However a cop can still do you for <35 in a 30.
 
ubersonic said:
From a speed camera, nothing (if ACPO guidelines are being followed by the local camera partnership). However a cop can still do you for <35 in a 30.
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This. Although for some time in London speed cameras were set at 42mph before they triggered a ticket but are now set at ACPO guidelines of 35.
 
Those signs are set to warn you of the speed limit, so are most likely set to a lower speed than the limit.

The speedo is not dead accurate, but they vary from car to car, a friends A3's is around 7 mph out at 70mph, but at lower town speeds is spot on :p
 
All the signs seem to be in sync going by me 1500rpm in 5th = 30mph real speed which is about 35mph indicated. Redline is around 6500 so topping out at 130mph real speed sounds about right.
 
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