Speedo Calibration after Tire Size change

Jman2025

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Switched from the stock square 225/55r16 to 265/35r18 (front) & 285/35r18 (rear) for my '03 . I'm assuming my Speedo is gonna be off after this, though I don't know by how much. I called the dealership but apparently they don't perform calibrations to vehicles this old.

For those dealing with a similar issue, how did you calibrate your Speedo?
 

white95

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Do you happen to have a SCT handheld for your car? You’d change the “tire revolutions per mile” and that would be that. Check your vehicle speedometer compared with GPS and adjust until you’re happy.

There used to be a “Ford Racing speedometer calibration tool” for the 99-04 cars but I’m not sure it’s available any longer.
 

96blak54

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Your old stock overall diameter was 27" while your new overall diameter is 25.8". Not a big significant difference, but will have an affect on the speedo by only a few miles per hr. If you are going to spend money on a device or someone to program to recalibrate the speedo, maybe maximize the outcome with a gear swap?
 

lutter94

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If his original tire size really was 225/55R16, that is a tire height of 25.75". The new 285/35R18 is 25.85". Which is 0.4% taller, 60MPH is now 60.2MPH. Can't loose sleep over this one.

27" sounds tall for a factory tire size......
 

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