I could be wrong, but I'm not sure that tire size is why your ASC is going off. I had 235-45's front and back until recently, and replaced with 225-45's up front and 255-40's in the back, and have never had the ASC light come on (unless of course it's doing it's job). Of course my current tire set up is ideal, because I only have a difference of .3" or 2 revolutions per mile which is pretty good considering the offset.
You might have your code read or do the pedal dance, to see if it's an ECU error.
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