I believe there to be a formula whereby you can calculate the mph per 1000rpm in any gear, provided that you know the gear and final drive ratios and the overall diameter of the tyres.
Does anybody know this calculation?
Purely as matter of nerdy type interest, I would like to check the speedo accuracy at M-Way speeds.
At the moment I tend to assume that 76-78mph is around a true 70, and wonder why most other are going so slow (except for the German cars of course).
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Sorry, can't help with the formula, but your question prompts me to wonder why people frequently query the accuracy of their speedo, but never the rev counter?
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the best way to check the accuracy of your speedo is to use the k/m marker posts on the motorway (or the police calibration markers if you know how to spot them and use them)
simply drive at an indicated 120kph and time yourself thro the km using a stopwatch.
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sorry forgot to mention, that should be 30 secs for the km
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It would be foolish to use the marker posts on the motorway. They are supposed to be 100m apart, but vary between 70 & 130 in my experience.
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I believe there to be a formula whereby you can calculate the mph per 1000rpm in any gear.
It's mentioned in this thread.
www.honestjohn.co.uk/forum/post/index.htm?v=i&t=10...1
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Speedos were required to be within +/-10% and in the 80's many manufacturers would configure to over-read by 9.9%. My Mk2 Cavalier SRi showed an indicated 134mph when red-lined in top gear (down hill, tail wind!) but I never thought this was any more than 122mph true.
Now speedos have to be within -0% to +10% but manufacturers now get fairly close to accurate.
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Thank you all, I shall unleash my spreadsheet on this.
Reminds me of the time nearly 20 years ago when I spent ages on a complex spreadsheet to calculate the gas boiler size for a heating system. It took everything into account including pipe lengths, room size, construction, insulation, number of floors, etc. etc.
The answer came out at (I think) 46K BTU, including 15% contingency.
Along came the Gas Man, looked round for a few minutes and said, "You need about 46K BTU, so I'll stick in a 60K"
Sometime we can be too clever...
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I have, in a previous job, been responsible for replacing motorway 100metre marker posts. I would seriously not bother trying to use these for any calibration purposes as many missing or damaged posts are replaced using just guesswork.
Also police calibration marks are not, as far as I am aware, spaced at any specific standard distance as this is not how the VASCAR system that uses them works.
PP
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Have found a site with all the calculations already in a page.
www.unixnerd.demon.co.uk/revs.html
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Triad the link, according to that at 70MPH I should be showing 2910rpm.
Took it down the A2 this AM andf at 2900 revs the speedo showed 70mph.
Spooky or what?
Repeat 3 times before each trip " the speedo on this car appears to be accurate, do not exceed an indicated 78mph"
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Even if a marker post is out by 30 metres it should not be too serious if you measure over at least one km, and do it at least three times to get an average value. The biggest source of error will probably be trying to maintain a constant speed while you take the measurements!
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In reply to PeterPerfects post, sorry but Police calibration marks ARE spaced at specific distances hence the title "measured mile / measured half mile"
These are used to check the accuracy of calibrated speedos and Average Speed recorders (VASCAR). If it takes 60sec to travel a mile at 60MPH then the speedo is bang on. There are of course slight tolerences either side. These checks are all evidential to corroborate the accuracy of the device.
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My information is based on the fact that the line painting crews I have associated with in my area have on more than one occasion been tasked to paint these calibration marks indiscriminitaly along certain stretches of road. Obviously well spaced, but nevertheless random.
Maybe this is not common practice, but it is I am afraid fact.
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When I talk about the poilice calibration markers, I am talking about round white disks on small posts on the side of the motorway. They are painted red in "quarters". ie you get 1/4 red 3/4 white, then 1/2 red 1/2 white, then 3/4 red 1/4 white, then all white. (could be the other way round)
I honestly thought that the km posts were accurate tho, at least over the full km or two.
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I guess my speedo is kaputt then!
I travelled 210 miles and it took 4 hours, but my speedo was always showing 100mph+ ;-)
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A505 between Baldock and Royston has these white rectangular blocks painted on the road, seemingly at random distances apart. Some only 300yds, some over half a mile, I guess these are to use with VASCAR?
While we're on the subject of markers at the side of the road (and I've seen the police measured mile ones, will try them next time) can anybody tell me what the reflective red and green diamonds I keep seeing on posts by the side of the road are? Red diamond or green diamond, about 6" high, about 5-6ft up a pole, on A-roads and motorways, all over the country.
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