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Premium Member
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283 Posts
Discussion Starter · #1 ·
Hi everyone,

My car again failed the SVA due to the speedo reading lower than my actual speed. I had it specially made by ETB Instrument using the cardboard disc test. I have done this test 6 times now and for each 100 turns of the wheel, the cable turns 82.60 times.They say that equates to 1299.90 cable revs per mile with my 225/60 15 tyres and as my speedo reads incorrectly, I must have done the test wrong as they have calibrated it correctly. It's a Jag IRS & RV8 auto.

Any ideas anyone as I'm running out of time and patience!

Thanks, Ady
 

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Premium Member
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Discussion Starter · #3 ·
that's a good idea, but unfortunately I'm not registered, taxed insured or anything and live in the middle of a town with 20mph speed limits and don't have a trailer to go and find somewhere legal to test it.
 

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Try measuring the circumference of the tyre with a tape, and see if that comes near to your distance calculation.

If you take the circumference without any load on the tyre, then you will build in a safety margin, as the actual rolling distance will be less.

If you’ve done the test 6 times and got the same result, then it’s probably OK, but you need to check that both of the rear wheels are turning together, unless it’s a locker diff, in which case they will be.

Hope this helps.
 

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Semi Proffessional Prat
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What ratio and is it LSD?
 

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Grown up? Never!
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Ady,
I don't know if this is still correct or not, but if you only want to pass the test and sort the speedo out later then you could always run a push bike speedo for the test? I've run them on and off for years and they read spot on! Also if you get it wrong just punch it a new wheel measurement and have another go, they should let you have a few goes to get it write. The best way to measure the tyre is to chalk mark the tyre and the floor roll the car forward and remark the floor when the mark touches down again and then measure that distance, If you fit the sender to the front wheel there was never a way to check if the reading was correct or not, so when they drove the car around they saw the numbers change and had to except that it is right, not sure if this is still the case, but I mate that built loads of 7 style roadsters always did that as it would read the road speed and not how fast he could wheel spin:D
Chaley
 

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Chaley, Adey could probably get away with it as he's being retested under SVA but pushbikes speedos , or any multifunction changing screen electronic units ,are not allowed on BIVA.
 

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Grown up? Never!
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Chaley, Adey could probably get away with it as he's being retested under SVA but pushbikes speedos , or any multifunction changing screen electronic units ,are not allowed on BIVA.
Bugger:sniff: looks like that acewell dash I've picked up isn't such a bargain:whistle:
Chaley
 

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Discussion Starter · #9 ·
thanks for all your help and good suggestions guys. ETB seem to think my wheel circumference should be 2.42m to make the speedo work correctly, when it's 2.16m, so I don't know what's going on there! They are going to try to adjust the speedo so it should read correctly on the re-test.Although I'm not confident this will work so I'm fitting a cycle computer and light which can be fixed over the speedo at the test centre if it fails again. Or I may just fit it before and worry about the speedo 'when' I pass!

L8rs, Ady:D
 

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draggin bumpers
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that's a good idea, but unfortunately I'm not registered, taxed insured or anything and live in the middle of a town with 20mph speed limits and don't have a trailer to go and find somewhere legal to test it.
good point I would never condone taking it out in the middle of the night down and industrial estate to get it right for the test. Good luck with the revised maths :tup:
 
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