speedo recalibration
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New User
Joined: Apr 2005
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speedo recalibration
HELLO, FIRST TIME USER. I HAVE A 1999 F-350 DULLIE 2 WHEEL DRIVE. I AM CHANGING THE TIRE SIZE FROM A 215/85/16 TO A 235/85/16. DOES ANYONE KNOW HOW TO RECALIBRATE THE SPEEDO. PLEASE POST OR EMAIL ME AT BPHD90@HEART.NET
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Joined: Apr 2004
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235 is indeed the tire width. And the second number relates to the height and it did not change, still 85. But that 85 is a ratio, section height = 85% section width. 85% of 215 is only 183, but 85% of 235 is 200. So the section height actually grows by 17mm. That's per side, so the diameter really increases 34mm, or 1 3/8 in.
diameter of tire (in) = section width (mm) X section height ratio X 2 / 25.4 (in/mm) + hub diameter (in).
diameter of tire (in) = section width (mm) X section height ratio X 2 / 25.4 (in/mm) + hub diameter (in).
