I run 17's so can't really help (lower profile tires use wider rims).

Anyway, between F-bodies it's been found that there is some variance. So some cars can get away with a little more tire than others (this is usually in the case of 255s vs. 275s). This can be due to manufacturing tolerances and sometimes simply due to previous body repairs. The best thing to do is measure your specific car.

If you don't want to buy the tool, you could use your existing rims as a basis to see how much clearance you have. A quick search should tell you how to calculate your current backspacing and with a little math you should be able to get the new spacing with a different rim.