Bewls Posted December 20, 2005 Share Posted December 20, 2005 It is usual for a Caterham speedo top read ALOT under? Based on working back from gear ratios, diff ratio, and tyre circumference (that I got off Yoko web site, so accurate to the nearest mm) and taking readings from the rev counter and speedo at a given speed, my speedo is reading about 12% UNDER! This was confirmed when a few nights ago I travel from Sheffield to Great Yarmouth in an “indicated” c155 miles, which is normally about 180 in the tin top! Always thought it seemed as though I was going faster than was indicated! I always just thought it was because I wasn't accustomed to driving Caterhams ❗ Joe Link to comment Share on other sites More sharing options...
oldbutnotslow Posted December 20, 2005 Share Posted December 20, 2005 My cars actual was reading about 9% under the indicated speed when on the rollers. This I put down to the size of the wheels/tyres I am using compared to the original Academy set-up. Sounds to me as if the Speedo is set up for a different tyre/ diff ratio. Someone with more knowledge will come along soon I have no doubt. Grant 😬 183 BHP of Black and 'Stone Chip' excitement. 😬 here Link to comment Share on other sites More sharing options...
nickdodo Posted December 20, 2005 Share Posted December 20, 2005 Joseph, I'm also not sure of my spedo reading. I have 185/70/13s Yokohama 21s and cant find the rolloing diameter on the website. Can yo give a clue to how I find them. Nick in the (1987) 1700 X Flow... Link to comment Share on other sites More sharing options...
J A T Richardson Posted December 21, 2005 Share Posted December 21, 2005 I looked into this some time ago, and the advice I ended up with was "TADTS". This was on a 1700 xflow with 4-speed gbox and standard 165/13 tyres. For the 4-speed gbox you CAN get alternative gears to go in the box to change from 22 teeth to 21 teeth, but I had the best one for my settings and it was still WAY out. Time to invest in a bike speedo, I think... (I did and it's GREAT!) Stinky toy driver - and proud of it! Link to comment Share on other sites More sharing options...
nickdodo Posted December 21, 2005 Share Posted December 21, 2005 JAT For a bike spedo you still need to know the rolling circumerance of the tyre. Did you calculate, measure or find some other means of working it out? Nick in the (1987) 1700 X Flow... Link to comment Share on other sites More sharing options...
Bewls Posted December 21, 2005 Author Share Posted December 21, 2005 The car in question is a 4 speed (straight cut close ratio Graham Sykes internals), live axle cross flow with 185 60 R13 tyres Nick: Go to: http://www.miata.net/garage/tirecalcold.html For 185 60 R13 tyres it gives a rolling circumference of 1734.8mm, which is more accurate than you could ever hope to measure it. Thanks, Joe Link to comment Share on other sites More sharing options...
Bewls Posted December 21, 2005 Author Share Posted December 21, 2005 Forgot to say: I know its a case of TADTS for reading over, but mine is reading under ❗ As in when my speedo says I'm doing 90mph, I'm actually doing just over 100mph ❗ 😳 Link to comment Share on other sites More sharing options...
sjmmarsh Posted December 21, 2005 Share Posted December 21, 2005 Joseph Mine reads almost spot-on up to 60 and then reads progressively under above that. It is about 7mph UNDER when I am doing an indicated 90 (as compared to the GPS speedo showing 97). Steve Link to comment Share on other sites More sharing options...
nickdodo Posted December 22, 2005 Share Posted December 22, 2005 Joseph You say... Nick: Go to: http://www.miata.net/garage/tirecalcold.html For 185 60 R13 tyres it gives a rolling circumference of 1734.8mm, which is more accurate than you could ever hope to measure it. This is the calculated circumferance. I understood that tyre manufactures have a dynamic circumferance - the actual circumfeance seen by the vehicle. Any one know about this. For example Yokhama give the rolling circumferance of a 205/155/15 as 1805mm, the calculation at http://www.miata.net/garage/tirecalcold.html gives 1905.4. Which is quite a bit differnt - about 5% approx. So my 185/70/13 are calculated at 1851mm - what is the reality? Nick in the (1987) 1700 X Flow... Link to comment Share on other sites More sharing options...
Bewls Posted December 22, 2005 Author Share Posted December 22, 2005 Nick: I did have details from yoko about the circumference of their tyres, but I can't find it now, I did however think that it would be more or less the same as the calculated though, thanks for letting me know this is not the case! I'll re-post on here if I find dynamic circumference Joe Link to comment Share on other sites More sharing options...
Tom_C Posted December 22, 2005 Share Posted December 22, 2005 Is this any help here? Tom Link to comment Share on other sites More sharing options...
nickdodo Posted December 22, 2005 Share Posted December 22, 2005 wow... Thanks Tom, that gives 185/70/13 a rolling circumferance of 1790mm , (calculated is 1851) 61mm diff (2 and a bit inches in old money) - so I'm not going as fast as I thought I was... Nick in the (1987) 1700 X Flow... Link to comment Share on other sites More sharing options...
Myles Posted December 22, 2005 Share Posted December 22, 2005 So my 185/70/13 are calculated at 1851mm - what is the reality? Why don't you just measure it using the standard bike-computer method? Line the tyre valve up at the 6 o'clock position on the relevant wheel, make a mark on the ground, push forward one revolution, mark and measure. I did try to measure multiple revolutions to increase the accuracy once, but the result came out to be exactly the same. It'll be accurate to about +/- 5mm in at least 1600mm and is certainly good-enough for bike-speedos to remain in step with GPS to well over the ton. FWIW, there was a significant discrepancy between my actual CR500 rolling circ. and the published one for instance. Project Scope-Creep is live... Alcester Racing 7's Equipe - 🙆🏻™ Alcester-Racing-Sevens.com Link to comment Share on other sites More sharing options...
nickdodo Posted December 22, 2005 Share Posted December 22, 2005 Myles, which bike spedo do you use? Nick in the (1987) 1700 X Flow... Link to comment Share on other sites More sharing options...
Chris W Posted December 22, 2005 Share Posted December 22, 2005 I compared my speedo indication to a GPS readout (TomTom SatNav) and I am travelling about 10% slower than my speedo indicates (ie: I am only doing 70mph when my speedo indicates upper 70's). I guess this is the best way to have it on balance! Chris 2003 1.8K SV 140hp see it here Edited by - Chris W on 22 Dec 2005 23:51:26 Link to comment Share on other sites More sharing options...
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