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Last night I drove past a vehicle speed indicating thingy at the roadside. My speedometer indicated 41 mph and the thingy 37 mph, which suggests that my speedometer reads 10% fast at that speed. Is it likely that the percentage error will be constant throughout the whole of the speed range?
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my speedo registers 2mph fast from 20-100mph according to sat nav
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Two cars have had a constant % error, my current one doesen't it varies.
The thingy is not guaranteed accurate either.
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Don't believe the speed thingy, we have a "slow down" one in a 30 limit that triggers at anything over 25mph speedometer speed. I don't use my satnav locally but my speedo is consistantly 3mph fast by satnav.
Last edited by: Old Navy on Tue 1 Mar 11 at 17:50
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>> that triggers at anything over 25mph
Who told you that? :-P
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>> Who told you that? :-P
>>
I am glad you realise that I have never been accused of holding up the traffic. :-)
I did test it over a few passes out of curiosity, now like everyone else I give it a good ignoring.
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A word of approval for ON here.
Surrey and even Sussex have these utterly idiotic illuminated 'slow down' signs that come on if you are doing anything over a ridiculous mimse. They drive me into a rage that may well pose risks to other road users.
These things cost plenty and have no purpose except to pollute the roadside visually.
Do the carphounds responsible for them think we don't know what the speed limit is? That we haven't seen the sharp bend road sign? That we are decrepit and incompetent enough not to be able to apply pressure to the brake pedal in good time?
What do they think? Personally I don't think they do think, unless their thoughts are to do with personal profit and loss for them and their corrupt, insolent carphound associates.
Arson of houses is a despicable crime. Arson of those things is the act of a hero. I wish I were younger and more foolish whenever I see one.
Tchah! (Spits long stream of tobacco juice onto head of nearest domestic pet, just like that geezer in that movie).
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I doubt the true speed was less than 37 if l'Es's needle was showing 41, so I'd be inclined to trust the thingy. My empirical observation is that the digital thingies are pretty accurate, whereas those that simply flash Slow Down are triggered at perfectly legitimate speeds.
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>> Is it likely that the percentage error will be constant throughout the whole of
>> the speed range?
No.
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One way to check (although it does mean trusting another instrument you have no way of calibrating) is to check the rev readings at different indicated speeds in the same gear - say 20, 30, 40 and 60 in third. That should tell you if the error is consistent or not; my money's on Not.
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EDIT doh
Last edited by: Focus on Tue 1 Mar 11 at 18:52
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I've checked the speedo of the CC3 against a few roadside devices.
My conclusion is it is fairly accurate, certainly up to 50 mph.
Too close to take any chances, so I do no more than an indicated 30mph in a 30mph limit.
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Did the third-gear test in the Volvo on the way home.
20: 1,200
30: 1,800
40: 2,400
50: 2,950
60: 3,500
Not has it.
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>> Too close to take any chances, so I do no more than an indicated 30mph in a 30mph limit.
Wimp. 35 is fine.
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>> Wimp. 35 is fine.
Heh heh ! Knowing my luck though I'd cough as I went past the camera !
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>> I'd cough as I went past the camera !
And knowing your luck, the camera would focus on the gob of phlegm hurtling forward at a scientifically-established 42.3 mph, sorry guv, Sing Sing innit...
Best to wear a balaclava really.
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>>Wimp. 35 is fine.
I thought you had let him off with that!
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...Wimp...
That's me.
I was bullied at school, too.
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>> One way to check (although it does mean trusting another instrument you have no way
>> of calibrating) is to check the rev readings at different indicated speeds in the same
>> gear - say 20, 30, 40 and 60 in third. That should tell you if
>> the error is consistent or not; my money's on Not.
Nice idea, but how are you going to check the rev counter? With the speedo?
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Easy: with my digital frequency meter. 2,400 rpm should sound an octave above 1,200.
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In that case, you can dispense with the rev counter and just use the engine pitch ;-)
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The speedometer on a car is not legally allowed to underread, the allowed tolerance is +0% to + 10%, in practice it is more like +4% to +10%. On a modern car with an electronic speedometer the percentage error will be more or less consistent at all speeds as it is programmed in by the car manufacturer. There will be some additional deviation due to tyre wear.
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Speedometer accuracy - nyx2k
