I've asked this same question before. 255/70/15 tires won't work with a 4/6 drop, they will rub. Many have done a 3/4 drop with the 255/70/15 drop. You would want to run a 60 series tire with a 4/6. I'm going with the 3/4 since I have crappy roads in new england, and may use my truck as a truck from time to time.