Is there a general rule of thumb as to how much clearance is needed between the tires and quarter panel lip? I just installed new tires on my cuda and they are a little too close for comfort. On the drivers side I can barely get a finger between the lip and on the passenger side I cant even fit a finger between the two. I don't know the measurement off the top of my head but I would say roughly 3/8" to 1/2" clearance on the drivers side and about 1/4" to 3/8" on the passenger side.

1/2" is my minimum. Can you roll your fender lip?

I have 1/2 or less on mine, seems ok i made sure i filed any sharp edges off the lip so if it rubs it doesn't cut anything, when i use wheel dressing on the tires every now and then i can see a witness mark probably from going around corners, but wipes off and you cant see any wear.

The lip is not all you should be worrying about. You want to be sure there is enough clearance between the outer wheelhouse and tire when the suspension is in compression. I have plenty of clearance at the lip but there is evidence of rubbing up on the wheelhouse ('69 B-body Plymouth)

I'm afraid to roll the lips and ruin the paint on the car. I shaved a little bit off of the quarter panel lips to get this clearance. The car has aftermarket quarters on it so it has the huge lips that were probably 1" to 1 1/2" at first. I shaved them down to get this amount of clearance and then I filed the sharp edges. I'm just afraid to hit a bump in the road or take a sharp corner and the tire will contact the quarter panel and end up buckling it or ruining it.

Your sidewall height plays the biggest role. What size rims/tires are you running in the rear?

Consider changing wheel back space or offsets. Did this successfully to obtain proper fender gap on our E Body when installing rear 275-60-15's on 8 inch wheels. 1/4 inch was all that it took. Pair of wheels on Summit for less than 250 dollars. Far better than chopping up a virgin body, or worrying every time you rounded a corner that you may slice your 300 dollar Polyglas GT's

The tires are 295/55 15's in the back with cragars that are 15x8 with 4.5" backspacing. I asked around on here before and there were people saying that they run these same exact tires on cudas and they had no clearance issues. I 'm thinking my problem is in the aftermarket quarter panels.

Two other factors is the type of suspension and driving style.

Some suspensions like 4 bar w rubber bushing move sideways alot in hard cornering, leafs too.

Panhard, independant styles are solid. Clearance can be almost nothing.

If you do easy driving,you also can run minimum clearance.