It's tough to nail down "ONE" largest size that will fit, simply because of the differences in body measurements with how these cars were assembled. In other words, a 275 60 15 "MIGHT" fit on one, where on another it will not. So if you're looking for the absolute largest tire that will fit in the stock location, it's best to measure. Plus, on the 67 and up Darts, you have that built in fender skirt that adds to some of the fitment woes. It's best to measure each individual case, so you don't end up wasting money.