Currently running 17in. Cobra R's 10.5 with 315's out back and 9 with 255's up front. sick of the R's looking at a set of saleens. i found a set online, but they are 9inch all the way around. with it being an 18 rather then a 17 and only a 9 inch wheel how wide can i go? and if i find some 10.5's for the back how wide can you go on 18's? thanks.