i have 20x8.5 all around with a 4/6 drop. i used to have 235/35 on all four. i cannot go taller or wider in the front without rubbing issues while turning. they barely clear as is and do rub if i hit a dip at a moderate speed while turning. the back on the other hand has plenty of clearance. with the new motor, i had ZERO traction so i decided to get wider tires but kept the relatively the same height because i didnt want too much of a rake. I believe it was 295/30?? Those fit just fine under there. could probably have gone 305 and maybe 315 but my 8.5's are too small for that. i was pushing it with the 295's which ultimately led to premature wear due to them being mushroomed which caused cracking. lesson learned.
Traction was still not good due to the short sidewall so i sucked up the slightly raked look and went with 255/45 which is what i have on it now. look at the pics and you can see the clearance i have. sorry i forgot to take pics of the front. ill try and do that later. BTW these rims have a 14mm offset (1/2in)
Since you want 18's, you should have more width sidewall options to play with. just run the numbers on sidewall height to relate to what I have given you and you should be able to get a pretty good idea on what you need and how it will fit.
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This last picture is when i had the 235/35's all around when I first got the wheels.