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Keith Wallace
United Kingdom
367 Posts |
 Posted - 03/04/2011 : 07:53:37
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Just repaired and refitted the original speedo to my PB, but I am running on 450 x 19 tyres, how much allowance will I need to make to the speedo readings?
Many thanks
Keith |
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Bob Stringfield
United Kingdom
854 Posts |
Posted - 03/04/2011 : 09:15:56
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Easy - just stick a Satnav to the dash or take an iPhone and compare the electronic and mechanical readings. No power connection needed for this application.
IMHO the steering improves on tyres of the original size; The above test is still advisable if you live near speed cameras.
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graham holdsworth
United Kingdom
424 Posts |
Posted - 03/04/2011 : 09:48:11
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Keith
I also have 450x19 tyres on my PB. I did the calculation some time ago and you need to multiply speed by 1.09 ie at4000rpm speed increases from 59mph to 63mph approx. This is based on my Michelins having rolling diameter of 30in o/d.
Someones bound to shoot me down on this!
Graham PB0602 |
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graham holdsworth
United Kingdom
424 Posts |
Posted - 03/04/2011 : 09:57:13
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Keith
Sorry, I'm talking rubish. My rears are 475s not 450s !!
Graham |
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George Eagle
United Kingdom
3238 Posts |
Posted - 03/04/2011 : 18:07:38
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I am not 100% certain but I think this subject was covered in an old Yearbook. It starts with measuring the distance covered with one revolution of the tyre.
The SatNav solution is the easiest solution!
George. |
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Peter Scott
United Kingdom
1240 Posts |
Posted - 03/04/2011 : 18:32:34
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 quote:
Originally posted by George Eagle
The SatNav solution is the easiest solution!
George.
err perhaps the easiest solution is to fit 4" tyres. Then not only would your speedo read correctly but your car would look right as well 
Wow was I really that brave
Peter |
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sam christie
United Kingdom
3099 Posts |
Posted - 04/04/2011 : 09:45:12
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In the absence of a SatNav or a rev counter another obvious solution is,on a quiet road, to travel with an assistant in mobile phone communication with a second assistant with a driver in a modern car driven at a steady speed. When it is clear both cars are moving at the same speed your assistant simply notes the speedometer readings of both cars.The test is repeated at different speeds.
Sam |
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Bob Stringfield
United Kingdom
854 Posts |
Posted - 04/04/2011 : 14:38:05
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For an even more complex solution, try assistants with stopwatches, starting pistols and measured distances between markers.
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sam christie
United Kingdom
3099 Posts |
Posted - 05/04/2011 : 10:18:40
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Joking apart,and Bob I am sure you are only teasing, the method I describe was in reality very quick,ultra simple,as accurate as a modern speedometer and a family 'happening' which I thought was fun.
I discovered my speedometer was accurate at 30mph and harmlessly inreasingly optomistic with higher speeds.
Sam |
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Bruce Sutherland
United Kingdom
1564 Posts |
Posted - 05/04/2011 : 13:12:58
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A simple speedometer calibration check for road use where I suggest accuracy is required primarily for 30 and 60 mph; just use a quiet stretch of road with known mile posts (phone boxes on UK motorways are accurately positioned), and time between – most mobile phones now have a stopwatch facility................... 60 secs at 60mph & pro rata – easy!
Bruce. (PB0564) |
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