The car came with 17" in the front and 18" in the back. tire sizes are 235/40-17 and 235/40-18. Using an on-line tire size calculator, that adds up to a total difference approaching 4%. Do you think this could be the problem? What system/parameter of the cruise control is being influenced by the difference?

I am planning on replacing the tires eventually with 235/45-17 (stock) front and 265/35-18 back (the height is almost identical.) The rims are staggered width and the rears are WAY to wide for the 235 width tire - the edge of the rim is out past the side of the tire. It has already been curbed a few times and even being careful, I don't trust myself not to do it again.