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tire size and speedo error

fordgtfordgtMember
My speedo on my 74 is running about 20 - 25 mph at approx 65-75 mph higher than it should. I have been reading the posts about speedo error and have a slightly different twist. I had the tranny rebuilt and a new speedo cable put in. I also have changed the wheels/ tires. I was running 255 / 60VR15 with a 26 3/4" diameter. I am now running 285 / 40 ZR 17's with a diameter of 25 1/2. Is this diameter difference enough to account for 20 - 25 mph gain? If so is there a procedure published somewhere so I can take it to a speedo shop to get it recalibrated?
thanks
Gary

[This message has been edited by fordgt (edited 07-10-2002).]
Original Post
LB Lloyd Butfoy (Guest)
I would call a good speedo shop that has a rolling dyno, they can regear the speedo gauge to the right speed for less than a few $$ I know there is a few in L.A.that can do it.You should not have more then 10mph with the tire change, I would also check to see if your 90 deg. drive on the ZF has the right number to match the speedo. If you have a 1.1 ratio on the 90 deg. you should have a 1610 rev speedo or you should have a 1.1:1 90deg. to a 1210 rev speedo
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