Posted already mid-thread in a general discussion , then realised it should possibly be here.
I suspect my speedometer, as is quite common, is over-reading (i.e. displaying higher than actual speed). If I'm correct, I understand its illegal for a speedo to under-read, so most manufacturers err on the high side for safety.
However, I think it's more than a little high, and I'd like to check how far out it is.
I remember a method of checking using motorway distance markers (can't remember enough details to do it, though) but would it be possible/easier on a rolling road?
If so, what type of organisation would do this?
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We used to use a Leitz 'Corrovitz' (not sure of spelling). It bolts on to the car and shines a laser at the road surface. The laser speckle pattern is then put through an FFT filter and after some signal processing you get a very accurate speed readout. They used to cost about £5k, but now there are other manufacturers on the market and they are much much cheaper. As race-track operator might have them to borrow/hire.
There are also GPS based systems that give VOG (i.e. like speed camera warning systems) but I don't know how accurate they are.
IIRC we could always rely on Ford to be 10% fast. Most other speedos were around 5%.
Modern speedo's generally use an electronic transducer in the gearbox and a 'cross-coil' meter movement driven by a microprocessor. These are generally very accurate and so it is the manfr. who decides how much the unit should over-read - in the old days with cable-driven units there was a lot of variation from one speedo to another.
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Two ways of doing this
GPS. Borrow a recent GPS device (ie garmins), most will give you a speed reading. They are more than accurate enough (20 yards over 1 minute with a good number of sats on lock)
Motorway marker posts, set at 100 metres. drive at indicated 120kph and should should count ten (they are numbered) (ie 1km) in 30 seconds with a stop watch. (It has been pointed out on here that they are not always accurately placed tho)
But why bother? Drive at the speed limit and you know you are safely within the limit.
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The GPS I used didn't strike me as too accurate when speed was changing - slow to respond. Maybe it would be better at a constant speed, but I wouldn't have relied on it.
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HGV tachographs and speed limiters have to be checked regularly and recalibrated at 6 year intervals. When ours was last done it was a very quick procedure. Since I didn't take it, I've never seen the rollers that are used. Does anyone know if they are the wrong spacing for a car tyre? If not a friendly testing station may be able to help.
Our tacho normally shows that the self appointed speed police (those with too much time on their hands) are actually doing about 36mph in 40 limits. VERY annoying when you're between gears with full GVW!
As Renault family points out though, relying on the existing speedo reading does give a better margin for error if you want to avoid speeding.
Chris
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Aprilia and RF are spot on (as they invariably are).
My 1997 Garmin GPSII is very accurate and shows up speedo discrepancies very well.
If you think your speedo is out, take a really close look at your tyres to check whether they are exactly the same size as standard fit.
Even incorrect tyre pressures will alter the accuracy of your speedo, albeit by a tiny amount.
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I do believe, (dont quote me on this!) those road-side digital "speed check" signs maybe about right, when ever I've approached one, speedo showing say... 35 mph the sign flashes 31 mph at me, indicating to me that i'm doing 31 mph and my speedo is over reading by 4mph. Also approached one very early hours in the morning doing indicatated 40 mph as per speedo but sign flashed 36 mph...............
Hope it helps
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A rough and ready way to check speedo is to follow HGVs on a main road they are fitted with speed limiters set at 56mph and these are very accurate. Check your speed against them - my Escort indicates 60mph when doing this - obviously you have to do this a few times as HGVs don't always travel at max speed.
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A rough and ready way to check speedo is to follow HGVs on a main road they are fitted with speed limiters set at 56mph and these are very accurate. Check your speed against them - my Escort indicates 60mph when doing this - obviously you have to do this a few times as HGVs don't always travel at max speed.
Accurate? you have got to be joking. Had to do 85mph to get past 35 tons of "limited" HGV downhill on the M1 yesterday. And that was with with a "65mph limited" 68 seater coach up my boot.
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It's also clear that Irish trucks (and presumably other countries' too) are not limited to "60"mph. Not sure whether they are subject to our limit while driving here?
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The GPS I used didn't strike me as too accurate when speed was changing - slow to respond. Maybe it would be better at a constant speed, but I wouldn't have relied on it.
No they are not accurate for changing speeds. They use time/distance to get a speed, so variable speed will give you spot averages. But for speedo calibration they are perfect, Travel at a constant speed for 1 minute and you will have a very accurate (to within 0.5mph) reading
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That's what I'm trying to establish - exactly what speed I'm driving at, not what the speedo reads!
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>> Modern speedo's generally use an electronic transducer in the gearbox and a 'cross-coil' meter movement driven by a microprocessor. These are generally very accurate and so it is the manfr. who decides how much the unit should over-read ..
That probably explains why when I'm doing X mph on cruise control according to the speedo, the speed function on the trip computer usually shows X minus 7-8% mph - which I've always thought to be a more accurate figure.
If you have a trip computer you could try this - find a nice flat section of M'way, hold your speed at, say 60mph, reset you trip computer and see what the speed says...
It may not be 100% accurate but should give you an indication whether the speedo is out by a big margin.
Chad.
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Get out your OS map for your area. Find a flatish straightish 3 mile section of motorway or dual carriageway. Find two easily distinguishable features about 2 to 3 miles apart (bridges are an excellent reference point. On a quiet day, drive between these two points at a constant speed and time it. Now measure the distance on the map (even better if you can use 1:25000s) and use this and the measured time to calculate the average speed. Compare this with your speed reading.
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Maths isn't my strongest point, how much difference is there in speedo readings between a new tyre with full tread, and an on-the-limit tread?
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doug - as I posted a day or so ago on another thread, about 1½-2 %. A new tyre has 7-8mm of tread and a well-used one 2-3mm so the difference is about 10mm on a typical wheel diameter of 550-600mm. Put another way, that is a difference of 1 mph at 60.
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