New Rim/Tire Speedometer Question


New Member
Feb 17, 2010
Good Evening, I am new in the SN95 Mustang world and have a question concerning how to adjust the speedometer for a new rim/tire combination I recently put on my 2002 3.8l Vert.
I went from the stock 225/55/R16's on all four corners to a staggered set of NITTO 245/45ZR17's on the front with a set of 315/35ZR17's out back. I also have a SCT SF3 that I plan on using to adjust for the swap.
My question is how to come up with the proper tire adjustment figure to input into the SCT, or does this even matter?
Any help would be most appreciated. :nice:

PS. I understand how to figure out the adjustment rate for a set of 4 matched tires but not for the staggered set.
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Thanx! After some research that was my conclusion also. Using a tire calculator for each size, at 60 mph on stocks it converted to 59.9 mph on the 245's and the 315's as well. The difference in rim height is offset by the difference in sidewall height. I appreciate your help!
FWIIW; since the Mustang is rear wheel drive, changing the tire size on the front will have no effect on the speedometer.

Note: that if the front/back tires ARE different diameters, that difference has to be within 4-6% or the ABS light will come on. This is due rotational speed difference being beyond what the ABS system can adjust for.
Thanx for the input! The difference between front/back is within those guidelines because I do not have the ABS light on. (one less thing to worry about!)
I understand that the speedometer registers from the rear axle, my question was adjusting my SCT to the new wheels. The SCT can adjust the speedometer by either increasing or decreasing the revolutions per minute data to account for bigger or smaller tires. SCT's formula to determine RPM is vague at best so I wanted to be sure I was properly adjusting the programmer.
Turns out that I am dealing with a 2 RPM variance so no adjustments were necessary! Thanx again for your help!