101 Ambulance stolen - Warrington

Same idea here - except that i built the car pc 2.5 years ago or so, but it never made it into the car as it works as a nice touchscreen stereo which is ideal for when you have a big job on in the garage! :)

"Calibration" is dependant of the rolling radius of your tyres, so you travel further and go faster if the pressures are low. B-)

"Measured miles" used to exist near most police forces traffic HQs for checking their speedos etc. The one I used to know about, on the A69 Newcastle side of Hexham, disappeared a couple of years ago though.

Failing that, marker post on Motorways are 100m apart. You do the maths for miles and how many you need to pass in how long for a given speed.

Get a TomTom satnav and tell it to show speed - it's a 1 second average, but if you average it over a few (Like 10) seconds you'll get a decent calibration on your speedo.

P.

All good, but not really what I'm after. I want to check the accuracy of a device over about 1000 miles, on a mixture of roads. Currently I have a discrepancy of about 8% between the box and the Navara (the Navara being higher). What I don't know is whether one is low, one is high or a bit of both.

Rolling radius of the wheels can be maintained over that distance - wear will be negligible. The 1000 miles will be covered in 48 hours quite easily so pressures can be held static (I've just passed 5000 miles and have had the truck since end of March).

On or around Fri, 19 May 2006 10:56:58 +0100, Tim Hobbs enlightened us thusly:

the motorway posts have numbers on 'em too - you could pick a suitable bit of motorway driving, hit the trip meter as you pass a post with a known number and then drive e.g. 100km or something (or more, if you're going further on the same motorway) and then note the reading. In fact, for better accuracy, you could pull onto the hard shoulder, park beside a post, zero the trip, then drive, then pull onto the hard shoulder by a post near your exit point, and note the mileage on the truck milometer as compared to the distance on the m-way markers.

if you can do about 200Km on the same road, the measured distance should be accurate to well under 0.1% - the markers are 100m apart, so over 200Km the distance between markers is 0.5% and even if they markers are not measured with pinpoint accuracy, on that sort of a scale you can get well under one tenth of a scale division[1] accuracy. I'd have thought that would get you close enough, really. you should be able easily (especially if you repeat the exercise on several different motorways) to get the accuracy down to

0.05% by suitable statistical analysis, maybe better than that. You can position the truck within a metre of the marker post, I reckon, easily enough if you stop on the hard shoulder beside it... [1] cue discussion about accuracy of readings: when I was doing physics in uni, the bloke teaching us about experimental technique held that if you have a scale with divisions n, and measure a quantity x using said scale, your accuracy was always +/-(n). I held than in general you could easily get +/-(0.5n). There are special cases where this doesn't apply, but as a general rule, your starting point will be chosen to match a scale division to high(ish) accuracy.

Suppose, for example, you measure a steel bar, which has an actual length of

123.4cm using a ruler with 1cm divisions. You choose to put the ruler so that one end of it (or a chosen scale mark) lines up as closely as you can get it to one end of the bar. you then judge the length by looking at the other end, which falls between 123 and 124 on the scale. In this case, it falls somewhere approximately half-way between the marks, and therefore you can quote the measurement as 123.5 +/-0.5cm. The lecturer bloke was adamant that you'd quote it as 123 +/-1cm. If the reading was obviously a lot nearer to 123 than to 124, you'd quote 123.0+/-0.5cm. You can argue that you have an error at both ends of the measurement but for the vast majority of measurements, you wouldn't do that, you'd move the scale and reduce the error at the starting end to a minimum. Even if you can't actually do that, for almost all scales, it's possible to judge to the nearest half-division by eye.

Austin Shackles uttered summat worrerz funny about:

I'll do some digging here but basically those vehicles we use are sent away and come back with a calibrated Speedo and certificate, theres also a sticker and serial number on the dash. We then run checks on the vehicle either weekly or certainly prior to any operation to target hot spot areas. These boil down to driving a measured mile (marked on various road surface in the county with a white rectangle in staffs - usually Dual carriageways, not round circle markers though they are just used for VASCAR) at 60mph against a stop watch. Also running through the laser at both 30 and 60mph to then check against laser and vehicle. The lasers are seperately calibrated and tested.

I'll find out te address for you.

Lee

I walked a measured mile, and it took me 14 and a half minutes, and then I set my pedometer by it, and discovered more recently I had covered a mile in

12 minutes and that was not a straight mile downhill like the one I used for calibration either and included crossing roads wierd to say the least

I walk with a cane and a painful back and all

Not that I believe a word of that, its like those speed warning signs, you have to take into account the curvature of the earth the tides, the influence of cosmic rays, the density of rubber, the astrological alignment and whether you were driving along a ley line or not.

You'll be telling me next they have solved the problem of longitude and there is no bermuda triangle :)

|| I had || covered a mile in 12 minutes

A mile in 12 minutes? That's *jogging*.