I noticed when I went through one of those speed traps that flashes up your speed on a board that at a speedo indicated 30 mph, it said I was doing 28mph.
Thats fine, but my dad's TomTom which also tells you how fast your going, says that at an indicated 30, im doing 26, so which is most likely to be right?
The differential between what TomTom says and my speedo says gets higher the faster I go which I assume would also happen with the speed trap, but which do I trust?
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Does it matter? As long as the speedo says 30mph, you cannot be done for speeding in a 30mph or higher zone.
Also remember that you are merely glancing at a needle for a microsecond, rather than reading two digits (which whilst not necessarily more accurate) at least give an instantaneous and specific readout which you can check against the TT or roadside sign.
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Does it matter? As long as the speedo says 30mph you cannot be done for speeding in a 30mph or higher zone.
I would not care to put money on that. Admittedly most car speedos err on the generous side, but having said that it is only a machine, or in many modern cases , an electronic gadget. Furthermore having an inaccurate speedo is not only inadmissable as a defence (or even a mitigating plea) it is AFAIK an offence in itself.
I trust car speedos like I trust fuel gauges; not TOO much!
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(TomTom or speed trap) which do I trust?
"But my "tom tom" said I was only doing 96, Officer, honest!"
"Oh, alright, I'll come quietly..."
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All speedo's since the mid nineties have a spec that allows it to over read but not under-read. GPS devices do not increment in 1 MPH unless you have a device like a Trimble Differential GPS costing £2 to £3K thus the confusion over 'real' speed. If you very slowly increase your speed you may note the your 'Tom Tom' or other device jumps in 2 or 3 MPH steps. Note when it clicks up you have exceeded the new indicated speed, so basically deduct 2 MPH and you will be very close. So a 29 to 31 could well be a 30 real ground speed. I have travelled over a 1000 miles with a differential GPS and a PDA GPS side by side and I can Assure you this is the case. My Audi A6 has a speed warning option on the speedo but it does not use what is indicated but what the ECU thinks is correct, at 71MPH the alarm goes off and the speedo reads 76 the Differential GPS says 72 mph so that is close enough for me. Take care, having a log from a GPS in not a defence in a court of law when the summons come through your door. Regards Peter
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It was just curiosity. I dont mind my speedo over-reading at all, its a good margin for those times the speedo creeps up past the limit. I was more curious whether the over-reading on my speedo was why I was trundling along at 60 but with many cars mounting up behind mine on an A-road.
Also at an indicated 70, I seem to pass lorries much slower than in my newer van, making me think that my Rover's speedo is a bit behind other cars.
My mums Hyundai, also using the same sat nav unit, shows 28 mph at indic 30. At 70 indic, its showing 68 rather than the 64 my Rover shows.
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The accurate one belongs to the police-doesn't matter what you think!!
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>If you very slowly increase your speed you may note the your 'Tom Tom' or other device >jumps in 2 or 3 MPH steps. Note when it clicks up you have exceeded the new indicated >speed, so basically deduct 2 MPH
My tom tom speed indication increases and decreases in 1MPH steps.
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So does mine!!!
In fact i have had several TT's including the first 'Navigator' software the that run on a PDA quite a few years ago.
All of them have had increments of 1MPH.
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The Tom Tom is likely to be far more accurate than your speedo.
These 'speed trap' devices are set apparently at random, or perhaps at the whim of the local 'speed police'. Went through several in villages near the south coast a while back, and got fed up with seeing them flash at me despite having less than 30 on the speedo. Checked speed with the TomTom on the return leg and found some were still triggered when speed was as low as 25 (and yes, the limit was 30!).
It struck me that this was self-defeating. Anyone using the road regularly will simply ignore them if they are going to trigger at very low speeds. Their benefit in warning those exceeding the limit is lost.
JS
Edited by John S on 10/01/2008 at 20:00
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I have found these signs to be very centre-weighted to the prevailing limit, eg, at ~30mph, they show 30mph, at 45mph, they show 51mph and that sort of thing. I often speed up just before them as I don't like them.
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This road sign I was talking about actually displays your speed rather than just tells u to slow down.
I think my speedo over-reads by about 5 mph at 70 odd and prob 2-3 at 30.
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The TOM TOM or any other GPS nav system should be near perfectly accurate. Both of my A4's showed 80 mph at an indicated 85 mph (on foreign roads of course ), whilst both my A4's were indicating exactly 30 at 30 MPH on the sat nav.
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At the risk of being pedantic, it's the "real speed" that's accurate...:-)
I pass through one of these speed advisory signs on a regular basis and the display figure is different almost every time, despite the fact that I'm doing an indicated 50mph, the speed limit for that particular stretch of road (you are supposed to slow down to 40mph soon afterwards for what is quite a modest bend).
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Might I respectfully suggest that those people who are so carefully observing the increments on their toys should be paying a little bit more attention to the road ahead? ;-)
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Might I respectfully suggest that those people who are so carefully observing the increments on their toys should be paying a little bit more attention to the road ahead? ;-)
No you may not.
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