this question comes up alot. people want to know the widest tire possible but you cant always go by number of what people use (i.e. 245,255 and so on). tire companies dont have the exact same sizes, some can be up to a half inch difference and both say 245 on the tire. i went with a low profile set up and fit 255/40/18 on my 68 dart and i grinded off the inner quarter lip a little just to be safe. id say your best bet is to pick what exact tire you want first and i know certain tire shops can type in the tire and size and show you the exact size of the tire in inches. also the rim width will change how fat the sidewalls get. or see if one of your car buddies has a similar set up that you want and ask to throw the tires on your car to see how it fits. hope that helped, goodluck with it