I have in my mind that it is legal for a car's speedo to read up to 10% above actual speed, but not below.
Would agree that car's speedo's have improved - from my sample of two cars, the '95 Cavalier has a far more optimistic speedo than the 2001 Mondeo!
My Father (scientist) worked out from gear ratios and wheel diameters what 70mph was in his Mondeo - he therefore cruises at a consistent engine speed, and apart from tyre diameter variance, he knows exactly how fast he is going (not very, usually!).
Having sat (frustrated) beside him on a number of occasions, and also sat in a friend's A6 cruising considerably faster with a GPS kit (reporting 'true' speed), it was (mildly) interesting to note that while travelling at a constant speed (according to RPM or GPS), the speedometer will indicate quite a wide range of speeds. Speedo's are optimistic, but not consistently
Jono
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while travelling at a constant speed (according to RPM or GPS),
How do you know if a rev counter is out though, if say a car has a gear ratio that says in fifth it has 20mph per 1000rpm and you drive with the rev counter at 3000rpm but the speedo says your doing 70mph, is the speedo out? is the rev counter out? are both out? and if so by how much?
ooh my head hurts now.
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I've posted this previously:
I used to work for a car manufacturer and there's an EU rule (type approval reg or something similar) that says the speedo has to over-read by 4KM/hr (which is 2.5MPH) and can over-read by up to 10%. This is at all speeds.
Tyre wear has a negligible effect (it reduces the diameter by maybe 12mm) but I can never remember which round the difference is.
Above info is getting a bit old now, so if it has all changed I'd be grateful for the correction.
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Thanks, Vista1100. I thought I'd seen it here. May I quote you on the club forum?
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My Audi is 3mph out at all speeds. Rev counter is spot on - 26 mph per 1000 revs in 5th = 78 mph at 3,000 revs - checked by GPS.
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I don't have GPS - how do they work out your speed?
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Espada III - well if you have a family and need a Lamborghini, what else do you drive?
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Doesn't it work out yoru speed based on your distance, speed - whatever from the satellite?
I know it's something to do with the satellies!
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Adam
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How does GPS calculate speed?
Well along witht he GPS signal telling you where you are (to about 5 to 10 yards with good satelite coverage above about 7 satelites in view) there is also a clock signal sent.
So its an easy matter to caluclate distance covered/time and provide very accurate speed to (as long as its a constant speed in a straight line over about 200 yards minimum)
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speedo has to over-read by 4KM/hr (which is 2.5MPH) and can over-read by up to 10%. This is at all speeds.
Vista,
Is this why car speedos don't seem to marked to read any speed lower than 10mph, because of the way the dials have to be 'twisted' anyway to read 3mph when stationary? I presume so but I've never really thought about it until now!
The one thing I can't understand about speedos is why people complain when they get a speeding ticket that "I was only doing 32mph according to the NIP" and "I always drive at or below the speed limit" yet there speedo would have told them they were doing 35mph.
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Vista, Is this why car speedos don't seem to marked to read any speed lower than 10mph....
They're not twisted as such, it's a calibration thing. When I did this the fairly crude electro-mechanical system of converting the speedo cable rotation into the speedo reading worked erratically at very low speed. It's probably done electronically now.
The one thing I can't understand about speedos is why people complain when they get a speeding ticket that "I was only doing 32mph according to the NIP" and "I always drive at or below the speed limit" yet there speedo would have told them they were doing 35mph.
I always think that too. Around here the police loan mobile illuminated speed displays to villages. I drive through at indicated 30 and they display 27.
I assume speedo variation is why many people on the motorway drive at almost exactly (but not quite) the same speed. So overtaking takes an age because they're going 1MPH faster. They probably all think they're doing 70.
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Thanks, Vista1100. I thought I'd seen it here. May I quote you on the club forum?
Well if you're going to quote something, then this is the best I can find quickly. It's from Hansard 12th March 2001. It's slightly different from my recollection (10%, PLUS 4km/h).
From: www.publications.parliament.uk/pa/ld200001/ldhansr...m
Speedometer Accuracy
Lord Allen of Abbeydale asked Her Majesty's Government:
Whether, in the light of the increasing importance of speed limits, they have any plans to make it easier for the private motorist to have his speedometer tested for accuracy.[HL839]
Lord Whitty:
The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed.
A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests.
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The speedo is calibrated when the car is built so who's to say that 5, 10 years down the line it is still over reading? Having nowhere you can legitimiately test your speedo is daft. Makes more money for the government though. If it is going to be the case that every 1mph counts then it should be on the MOT so you know that it is reasonably accurate each year.
Old cable speedos can underread. Someone on another forum talked about their old triumph and moggie minors being wildly inaccurate, especially at higher speeds. I think they said at an indicated 70 the car was actually doing a higher speed!
I think the question of calibration should again be asked in Parliament as it is unfair that people get prosecuted for an instrument when it isn't accurate and more importantly whose accuracy you can't verify.
teabelly
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>>Having nowhere you can legitimiately test your speedo is daft.
Agreed - I was in Australia over Christmas and they had "complimentary" speedo checks - a police car with all the gear set up along a straight road - you had warning 1 or 2 k's up the road, set your cruise control and the radar told you the real story.
Only place I've been through a speed trap at an indicated 107 with impunity :o)
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Thanks again, Vista1100.
Presumably he meant to say "the indicated speed must not exceed the true speed by more than 10 per cent plus 4 km/h". Or have I not yet woken up properly?
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Thanks again, Vista1100. Presumably he meant to say "the indicated speed must not exceed the true speed by more than 10 per cent plus 4 km/h". Or have I not yet woken up properly?
I noticed that too, but I'm not going to edit Hansard :-)
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I always think that too. Around here the police loan mobile illuminated speed displays to villages. I drive through at indicated 30 and they display 27.
I drive past one regularly that tells me to slow down when I am doing an indicated 30 mph.
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I doubt whether they are calibrated to a narrow margin - a comment form on site Officers with the knowledge would be good.
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My Honda reads over by a consistent 5% compared to GPS.
Had a 51 plate pool Laguna last week and with 92 on cruise and speedo sat nav was reading 90.
This assumes that the Sat Nav speed calculation is accurate.
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Better optimistic than the reverse...:-)
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What\'s for you won\'t pass you by
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