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Speedo: Under reading


Bewls

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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

 

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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. 😬

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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!

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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...

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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 *thumbup*

 

Joe

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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...

 

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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

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