I think the speedo in my new car is overreading quite a bit at 30 mph.
Is there any way of getting this tested and/or re-set?
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By law a speedo must never under-read; the accuracy required SFAIK is that any speed it may read up to 10% over and 0% under. In my car, checked against a GPS, the speedo over-reads by 3mph at all speeds.
You can check yours by using or borrowing a handheld GPS, mine reads to 1/10th of an mph or, with a passenger's help ,check the time taken to travel between the mile markers on motorways. They are accurate enough for this sort of calculation.
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If the car is new as in brand new, then send it back to be fixed. That's what the warranty is for.
You could provide an estimate of the error by timing it against the marker posts on a motorway whcih are 100m apart. Most cars over-read by between 5% and 10%, in my expereince.
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Well yes, but he doesn't know by how much it is over-reading and I bet the garage have no means of checking it. I think the OP needs to be able to give them a figure and one that shows the speedo is illegal before he will get much joy from the supplying dealer.
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Are you sure that the markers are 100m apart on the motorway????
Something from my distant past says they are 176 yards apart!
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pmh (was peter)
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PMH I think you are right; 10 posts to a mile. Having them 100 metres apart would be illegal - Distances are not allowed to be diplayed in Metres, a sign saying Free Parking 200 Metres is illegal and long may it stay so!
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er no, there are ten posts to a kilometre. They are not in fact mile markers, or signs, but "marker posts" so they can quite legaly (and indeed are) be at 100 meter intervals and be counted in 10's.
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TourVanMan TM < Ex RF >
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TVM. I bow to your superior knowledge! I can't rememebr when I was last on a motorway and when I was I wasn't counting the marker posts! BTW what is the use of 100 metres apart posts in a country where speed and diistance is measured in miles? They are in fact only to indicate the direction and distance to the nearest Emerg Phone aren't they?
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They do have a little telephone symbol on them with an arrow to the nearest emergency phone, but primarily they are there for surveying, catographic, localising a specific place on a motorway, and road maintenance reasons.
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TourVanMan TM < Ex RF >
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TVM
OK, I stand corrected! But when did they change the spacing from 176 yards????????
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pmh (was peter)
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Some A-roads in West Sussex have white squares painted on them that are about 15 inches square and about 0.2 miles between pairs. You would need to ask one of the local patrol drivers as to the exact spacings of any pair you could make use of. The all you need is a stop watch and calculator.
I assume the squares are there to check their speed measuring gear. For instance, the A23 to Brighton has several pairs.
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The squares are actually part of the process/equipment to nick you for speeding. They time you over the squares ( or diamonds in some places). Sometimes there is just the one and another prominent feature like an overpass.
To actualy calibrate the gear on a motorway you will see other marker posts, usually a litte red square with a circle quartered in it.
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TourVanMan TM < Ex RF >
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The "squares" can also be used by police helicopters, to time a car over a distance.
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Might be worth trying a local tacho calibrating centre.
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"Might be worth trying a local tacho calibrating centre. "
That sounds good, I wasn't sure if such places existed.
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Yes there're tacho centers but only for LGV sites.
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Its not what you drive, its how you drive it! :-)
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The lines are calibration marks to allow a high-speed data machine to take readings of the road surface. The lines are there to make sure that the machine takes subsequent readings in the same places.
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Why not mark out a 30 mile route on a roadmap and see if it takes an hour to drive it?
V
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Hello Peterb, What car is it what is the original rim and trye spec ( size, with and J rim ) and what rubber have you got on now. Regards Peter
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Thanks Peter
Car is Lexus IS250 Auto (base model), supplied new.
Wheels/tyres are: 205/55R16, Bridgestone Potenzas.
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Can't you just beg steal or borrow a gps unit for haf an hour and compare the readouts? (Or would a gps unit be no more accurate in these circumstances?)
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saying Free Parking 200 Metres is illegal and long may it stay so!
I just wish we would make up our mind and go one way or the other instead of sitting on the fence. Nothing about the current "mix" of the two systems makes any sense whatsoever.
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"Can't you just beg steal or borrow a gps unit for haf an hour and compare the readouts? "
Isn't gps only accurate on the flat?
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Isn't gps only accurate on the flat?
In this case, yes. The GPS will quote the distance travelled as your "course over the ground" as you move across the surface of the earth. But most also give a read-out of height above sea level, so you can use this to double-check that you are travelling on a level stretch of road as you verify the speed read out by the speedo.
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Borrow a GPS nav unit is the best way.
I find with my via Michelin navigation system that the GPS receiver reads bang on 30 at an indicated 30 mph and bang on 80 at an indicated 85 mph.
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Peterb. But are those the default factory fit rims and tyres for the car. A GPS/Nav aid and a decent length of fairly straight motorway will soon tell. Regards Peter
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"But are those the default factory fit rims and tyres for the car?"
Yes
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