Usually it is said that most speedos over rate speeds. Does it affect odometer reading? How accurate are odos?
Also, if someone uses a different size of tyre, it will affect odo reading as well. That means even without clocking the odo, you can get a false mileage :)
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An odometer should be fairly easy to check using the trip reading which will probably indicate to a tenth pf a mile. Find the distance from A to B using an online route planner such as tinyurl.com/32bfz7
If you pick a journey which you make routinely it won't cost you anything to do the test. The longer and more straightforward (i.e without too many corners) the journey the better it will probably be.
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Google Maps tells me that my journey to Liverpool yesterday was 190 miles. My odo read 194 when I arrived.
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Are you asking how accurate speed is or distance ??
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Are you asking how accurate speed is or distance ??
Odometers measure distance, speedometers measure speed. (I think!)
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Back when car magazines used to test these things (1960s, 70s) it was quite common for odometers to read a percent or two fast. Not sure why this would have changed, if it's fast it makes it appear you've done more miles to your gallon!
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Odometers can't read "Fast". On my car the difference between the distance recorded on the odometer and on the computer system amounts to 1 mile in 100.
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I did a journey of 180 miles according to tomtom yesteday and my trip and speedo both increased by 180 as well so mine is inline with tomtoms predictions. The speedo does overread by about 5mph though so obviously no connection between speed and distance calibration.
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Well I took my 1930 odometer as being geared right when I repaired it and used it to calibrate the speedo. The speedo read about 50mph for a true speed of 60mph so was truly a liability. Set it to read about 64 for a true speed of 60 and comparisons with other cars suggest I got about right. (60 was picked as a mile a minute makes for easy maths etc).
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In my last car the odometer unwound if I drove backwards. Not sure with present car, it's a digital readout and I haven't checked.
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Don't put it on bloks and do otherwise you might end up like Ferris Bueller. Anyway unwinding in reverse, with the current price of fuel, might be an overly expensive way of doing things!!
Out of interest (or not) these campavans that drag a car behind them, does the towed car have miles piled on to it as well? I presume it does.
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so obviously no connection between speed and distance calibration.>>
Back in about 1990, I took delivery of my new company 2.0 Sierra GLS - which at the time was a pretty good motor - well, it was to me, anyway.
I took it down onto the A14 and was mightily impressed when it effortlessly achieved 130mph. I was less than impressed though when, at around 100mph, much of the traffic began overtaking me.
Twice it went back to the main dealer and twice the idiots replaced the speedo head etc but still it gave a ridiculous reading. I noted also that my journey to work was taking 11.8 miles rather than the 9.2 that it used to take. Had someone been stretching the roads?
Fortunately my local independent had the good sense to run it alongside another car for comparison, dug a bit deeper and discovered that the wrong gearing had been put in at the engine end of the cable during manufacture.
I was kinda disappointed that it wouldn't do 130mph any more, though!
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My car's odometer trip recorded 44.4 miles for a journey which the AA route planner says is 45.0 miles, and I'm satisfied with that degree of agreement.
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Our local Car magazine has an odometer over/under reading in %age terms in the final stats of road tests.
I'll look up twenty or so tonight, and post the results, so we can then get a highest/lowest, and mean figure.
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A trader friend of mine used to buy cars with accurate odometers and sell them with inaccurate ones, if you get my drift.
I remember him showing me his 'clocking' kit which was basically small bits of stiff wire bent to suit the speedos of various models.
Each piece of wire had a tiny piece of rubber slipped over the end.
'Scratched numbers are a dead giveaway,' he used to say.
The same trader tried to connect his electric drill to a speedo cable in a bid to turn back the miles that way, but it was not a success.
This was when all cars had mechanical odometers and when clocking was not thought quite the sin it is today.
Edited by ifithelps on 02/06/2008 at 17:18
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when clocking was not thought quite the sin it is today.
I don't remember people being particularly tolerant of clocking in days of yore. It was worse actually because cars didn't last as long or wear as well as they do now.
I reverse-clocked a Skoda I had because the speedometer had failed. When I put a second-hand instrument from another car in it its recorded mileage virtually tripled.
Edited by Lud on 02/06/2008 at 17:42
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Ok, checked through a few yesterday...
highest was 4.7 OVER, lowest was 3.1 UNDER, mean on the 10 I checked was -0.77%
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That's pretty good.
My round trip to work was 78 miles on my old motorcycle, 80 miles in a particular car and 85 miles on my "new" motorcycle.
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On a long trip yesterday I noted that when the on board computer reading changed to 200 miles, the odometer read 200.3. This was on a 307 so seems accurate enough, for me anyway.
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How much difference does tyre pressure make to tyre diameter, and hence odometer accuracy?
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on a recent journey i used my sat nav and whilst driving i checked the speed my speedo was saying against the sat nav.......it averaged that the speedo was reading 3 miles less then the sat nav, unless the odometer on my car (being digital) is a seperate system the reading will also be out on each journey
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Odos and speedos can never be 100% accurate due to the wear on tyres from new to used - call it 1cm in the tyre's diameter, expansion of the tyre's circumference under speed etc.
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