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Post by Deleted on Jul 16, 2007 16:04:45 GMT
Hi all, has anyone undergone the trauma of having their speedo recalibrated following a change in tyre size, please ? Have done a 'search' but nothing comes up, so..... My big Manual has some info on cable revolutions for a 3.9 axle ratio but, being a P5B, with a 3.54 axle, no details can be found, even in the Supplement. Am I being naive or, do I really have to go grovelling on the floor to mark the wheel revolutions whilst counting the number of times the speedo cable turns ? Would have thought that Instrument people would have a book of magic numbers for that purpose ! Any help would be appreciated. Regards, John A.
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Post by harvey on Jul 16, 2007 17:04:18 GMT
I've used Speedy Cables (now somewhere in Wales I think, google should find them) to recalibrate speedo heads after doing conversions, and have never found the procedure difficult. You can always calculate the cable turns from one ratio to another provided you still have tyres with the original rolling radius. Which is probably unlikely by now. Far easier to mark the tyre,and the floor, push forward on a level surface six revolutions (if I remember correctly) and mark the floor again,then repeat using the marks on the floor as the guide and count the number of times the cable turns. If you stick a piece of masking tape on to the end of the cable to make a pointer it makes counting easier.
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Post by Deleted on Jul 16, 2007 20:06:56 GMT
Thanks for that. I have sent mine off to Speedograph, in Nottingham. They have outlined a similar procedure but, I was looking for an easier route as I don't have that much space; besides, its one heavy piece of kit to push around ! Regards, John A.
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Post by johnwp5bcoupe on Jul 17, 2007 6:19:39 GMT
Jack the car up both wheels so car is level put a chalk mark at the bottom of the wheel turn it the required amount while someone counts the RPM on the cable measure around the circumference with a piece of string or a long cloth tape measure and X 6 or 3.14 X Dia or 3.14 X R2 and multiply by 6 for the distance
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Post by Deleted on Jul 18, 2007 8:13:42 GMT
Thanks for that John, I understand your logic but, in the absence of any magic book of numbers, it looks as though dearly beloved is going to help out. Fortunately, our drive is slightly downhill, straight and, should just about be long enough ! Will keep you posted. Kind Regards, John A.
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Post by Deleted on Aug 18, 2007 10:36:19 GMT
Well, I used the drive to roll the car down and count the wheel turns and, the turns on the speedo cable but reversing back up needed a bit more gas to overcome the rolling resistance - you can see what is coming can't you ! - yep, hit the garage wall and mangled the rear bumper and, a bit more besides ! Still, Speedograph have done a first class job on refurbishing the instruments - they look as good as new, just need to renew the cable now to overcome the jerky movement.....! You have to smile, don't you ! Kind Regards, John A.
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Post by Deleted on Sept 25, 2007 15:12:16 GMT
Hi all, Just to close this one down, have now fitted a new speedo cable and, the small "gear-box" on the side of the main gear-box and, all is well - a nice and steady reading ! With my PhD in hindsight, I would say that the main cause of the 'waving' was the small "gear-box" between cable and, the main gear-box. The cable itself was oiled but did not have any shiney spots due say, to a 'kink' in the cable. Hope you find this of use. Kind Regards, John A.
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