How tall of a tire is the best fit? My memory keeps telling me that 27" is the preferred height. Close to a stock size. I will be putting in a XV level 1 system, and it lowers the car, so, should I run a 26" tall tire? My plan now it to run 18" x 8 rims in the front, 18"x9.5" or even a 19"x9.5" rim in the rear. I am leaning towards a 295 series rear tire and side wall is dependent upon tire makers tire height. Up front I am planning on a 255 series tire, with side wall dependent on maker tire height. I dont want too wide of a tire in the front to dull steering feel. Most high performance cars are not running ultra wide tires in the front in order to get better steering manners. I am sticking with the given trend. My plan is to buy these
http://tsw.com/alloy_wheels_snetterton.php
Stock size depends a little on how the car was equipped, but 7.35-14's were stock for Challengers and they measure about 25.9". F70-14's were available with HD suspension, those are 26.2" tall. So ideally if you wanted to maintain original tire height specs you'd want a 26" tall tire.
Of course, those were mounted on 5.5" wide rims, so when you go wider you have to consider clearance. But a 255 series on the front shouldn't be a problem at all. On an 18" rim you'd want a 255/40/18, that'll be a 26" tall tire. Obviously there's a little difference by tire brand and model, but its not enough to change to the next profile.
In the back, you're going to have to do some work if you want to fit 295's. XV level 1 suspension does
not move the springs, so unless you get an additional hanger/shackle offset kit you won't be able to get a 295 under there. With the stock spring location a 275 is about as wide as you can safely go considering that not all the rears are perfectly centered on these cars. Now, you can get one of Dr. Diff's 1" relocation kits, with moves the hangers and shackles in 1". You'll have to move the spring perches on your rear axle, or get a B body 8 3/4 which has the perches in the correct spot for a 1" relocation kit. Once that's done, you should be able to clear a 295 if its centered properly. I have a B rear and 1" relocation kit on my car, and with my 275's I have just under an inch of clearance to my quarters and my springs. The tire profile of a 295 is a full inch wider than a 275, which should leave enough clearance to avoid rubbing.