tire size and speedo error

fordgtMember

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).]

7/9/025:54 PM

Original Post

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