ianhurley20
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| posted on 2/12/14 at 10:36 AM |
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Speedo calibration
Hi all. Probably a non runner but here goes.
I am building a Haynes roadster with Sierra rear diff (3.62 : 1 I think) probably with 195/50 x 15 tyres and a Mazda 1.6 engine and gearbox. The Mazda
would normally have a 4.1 diff fitted with 185/60 x 14 tyres.
I want to use all the Mazda electrics including the speedo. As I see it it will read 88% of the speed it recorded in the MX5 due to the diff ratio
change. Does anyone know how I can get it re-calibrated? (I have thought of printing off a new speedo dial altered to indicate correctly and glue it
over the original ).
Ian
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ianhurley20
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| posted on 29/12/14 at 04:39 PM |
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Ok I've done some checking and find that I have a 3.92 diff not the 3.6 that I had been led to believe. That means that my speedo should read
95.6% of what it did. I've got some 195/45 x 15 wheels and tyres so they should read 3.65% faster. That means I will end up with the speedo
reading 99.25% of what it used to. I am depending on the original being a little optimistic so hopefully it will not under read. If it was acurate
then at an actual 30mph it would read 29.775mph, not enough to tell the difference given the thickness of the needle (I hope).
I have a problem checking before IVA as I have no local roads to check against a GPS to get actual speed. So, I have a question to those who have done
IVA. How do they actually check speedo accuracy on the test?
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adithorp
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| posted on 29/12/14 at 05:36 PM |
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You should be OK at that. It'd be better to be able to check before but not easy legally.
At IVA they'll put the car on a set of rollers that have a calibrated speed readout. Then they'll run it to set speeds on the rollers
(30/50/70 I think) and check the speedo against the rollers.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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ianhurley20
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| posted on 29/12/14 at 05:51 PM |
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So my biggest error with them would be at &0 where it should be reading 69.475 if the original set up was 100% accurate - fingers crossed then -
thanks
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ianhurley20
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| posted on 29/12/14 at 05:51 PM |
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Doh! 70 - fingers wont work
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adithorp
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| posted on 29/12/14 at 06:02 PM |
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Yes, if it was originally spot on. They very rarely are though; Usually over reading by 5-10% in which case you'd be OK.
Your speedo has to read equal or higher than the rollers actual speed (max +10% from memory).
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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