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tire size and speedo error

fordgtfordgtMember
My speedo is running about 20 - 25 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
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bosswrenchbosswrenchMember
The tire diameter mismatch is part but not all of your speedo error; 25 mph at what speed? Note the odometer uses a different geartrain in the speedo so for instance the odometer may be zeroed with a 27" dia tire but the speedo will still be off a little. Finally, what year is your car? There were two different right-angle drives, tranny gears and speedo heads. Early cars had one combination, all others had another. It's possible to mix-&-match these and get the wrong unit even with the right dia tires.
fordgtfordgtMember
mine is a 74. the error is approx 65-70 speedo shows 85-90 at about 80 it shows well over 110. I had the tranny rebuilt by Hall a new spedo cable from Pantera East and replaced the face of the gauge with a white face (also from Hall). I was thinking about mounting my old tires and wheels this weekend and see what I get
thanks
gary
J jbrubaker (Guest)
i believe your problem is in the speedo. when removing the needle to replace the face plate the needle must be recalibrated when it is put back on. any good speedo shop will be able to do it for you but you will have to remove the unit for them. i paid about $20 to have mine calibrated.
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