Folowing on from threads on speeding and cameras which touched on the inaccuracy of speedos on most cars.
I've found that those automatic 'slow down' signs are triggered at exactly the speed limit for that particular stretch of road. When my speedo says 30 the sign doesn't come on but when it reads 31 or 32 it does. I've found this with various cars.
Cheers, Sofa Spud
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Spud, if you check your speedo carefully I think you will find that your indicated 30 will be a true 28 or even 27. I believe they are deliberately made to indicate 'optimistically' (this applies right up the scale, though the proportion may vary).
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My portable GPS system gives an accurate speed readout. I find that most cars have speedos that are 5-10% overly optimistic. The Fords I've had are always 10% over (maybe Ford trying to slow down some of those reps?).
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>>inaccuracy of speedos on most cars.
Glad you mentioned it. I posted inaccuracy of speedos some time ago to be 10%. I meant and didnt correct 1%. kinda gather its only expressing the fact you are speeding. Not certain whether they take a photo.
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Was mech1
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Like Aprilia, I check speed on my GPS receiver and find exactly the same error on my Mégane. I believe the GPS checks speed by sampling point-to-point and may not be too accurate an indication of road speed on a curve. No idea what the error is, though.
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The big dial in the middle of the Mini's dash is unnervingly accurate when compared with both my TomTom GPS and my Garmin eTrex GPS. No more than 1-2mph out at 80, er, um, 70mph.
I've always worked on the 10% margin so figured that indicated 77 was nearer actual 70, as it was in the Citroens and the current Clio.
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Lee
MINI adventure in progress
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It's only comparatively recently that speedometers became far more accurate.
At one time (in some cases they still do) the police adopted the policy that speedometers read 10 per cent fast on average and to ensure fairness added 2.5mph to any reading to gain a more realistic figure.
So, if your speedometer was indicating 80mph, your (virtually) true speed was a fraction over 70mph, made up of 70mph, plus 10 per cent, plus 2.5mph, which equals 79.5mph. For 30mph the speedometer reading would have been 35.5mph.
It provided leeway for motorists who believed they were speeding and getting away with it...:-)
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Most speedo's are about 8-10% optomistic at 30, so your indicated 30 is anywhere between 27-28 mph. At 70 mph most get more accurate - typically 5-10% optomistic. GPS indicates the RF laguna is 10% optomistic at 30 and 5% at 70 (and back to 10% at errrr 100)
So taking the original point given that the radars used on the warning signs are not calibrated and can be expected to be up to 10% out, and your speedo the same, we could be looking at a 6mph diffrence. (or not as the case may be)
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how accurate are the GPS systems you are all using to 'calibrate' you speedos? The GPS systems i've used (albeit not new models)are less accurate than triangulation when fixing a position on a map (even worse when in motion), and their elevation readings were always significanty wrong. Whenever i'm doing work which requires accurate measurements, differential GPS is the only way to get the accuracy. I guess the incar navigation stuff must be reasonably accurate nowadays, but i wouldn't trust it to measure my speed. What sort of accuracy do you guys find when travelling?
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Now that jitter has been removed from the GPS signals, and given newer receivers and decoders they can be very accurate with a good view of satelites (i can get 9 with a re readiating antenna). Speed accuracy probably to .5mph, Postional accuracy to about 10 yards (tried it on a triangulation post). Height accuracy absolute rubbish
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Slewing off topic here, but for some reason GPS isn't too great at the ups and downs. That's why our eTrex Vista comes with a barometric altimeter. For X/Y co-ords most units I've had have been pretty good *(to 10m anyhow) but with WAAS and the Euro equivalent, I understand that the accuracy is even better.
So, er, it helps you locate your car in a large airport car park. Nicely back on topic there
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Lee
MINI adventure in progress
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If GPS systems were to use sucessive position fixes to compute speed then they would indeed return grossly variable results. Most GPS measure doppler shift in the GPS carrier wave from one or more satellites allowing them to derive a velocity vector without having to worry about absolute position. Quoted accuracy is said to be a fraction of a metre/second which makes it more than acceptable for automotive usage.
m.
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It's an EU regulation that speedo's have to over-read by at least 4KM/Hr (2.5MPH) and max 10%.
I've always wondered what the max effect of tyre wear (ie reducing tyre diameter by, say, 12mm) is?
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a tyre with outer diameter 612mm, reducing to 600mm with wear, gives a 4% change. Smaller tyre diameters will change more.
its a ratio of the square of the diameter.
and tyre pressure will make a difference as well, not sure by how much though.
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I read often, only post occasionally
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Sorry, spikey, your maths is at fault. A circumference is a linear measure, in direct proportion to the diameter - it's not an area, which WOULD be as a squared relationship. So 12mm change, being 2% of 600mm, would cause a 2% apparent increase in speed.
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Hi!
I went through a 'slow down' on the southern edge of Bristol yesterday. I was doing about 32 (just before the 30 sign, I hasten to add) and slowing down - and the sign lit up. I reckon the signs must be set to come on if they detect a real speed of 27/28 - i.e. a typical speedo reading of 30!
Cheers, SS
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Oh for an edit button, brains not in gear thanks to a three hour drive home from Brum this afternoon.
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I read often, only post occasionally
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"and tyre pressure will make a difference as well"
Not to mention centrifugal force. Again, not sure how much, but the circumference will increase slightly with speed. Anyone who doubts this should stand behind a Class A dragster at launch time!
All of which begs the question, how accurate are the big dials in police cars? I'm assuming they are still connected to the wheels...
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My rev counter is very accurate at giving the current speed when in top gear! The car is quoted at 26mph per thousand revs in top. I tested this with my GPS - 2,000 revs 52 mph, 3,000 revs 78 mph - too many cops and cameras about to check 4,000! Speedo was overreading by about 3 mph in both cases.
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Vehicle construction and use regulations require a speedometer accuracy to be in the range -0% to +10%. The Association of Chief Police Officers (ACPO) have a formula which allows a vehicle speed to be 10% above the limit plus 2mph. Unless anyone else knows different......
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L'escargot by name, but not by nature.
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Er! The wheels aren't pefectly round either - what about the flat bit on the bottom.
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Er! The wheels aren't pefectly round either - what about the flat bit on the bottom.
The distance from the centre of the "flat bit" to the centre of the wheel is the "rolling radius".
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L'escargot by name, but not by nature.
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JBJ, I would imagine that police car speedo's are checked in a similar way to HGV tacho checking and recalibrating. Tyre size measured, strobe on tyre sidewall to measure wheel rpm, known speed of rolling road. I think licensed taxi's also have to undergo a similar taximeter / odometer check to ensure that the taximeter charges accurately.
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