Fenders eating up tires...

I've got 245/35/20s on some 20x8.5s with 5.5" backspacing on the blazer in my profile. When I'm aired up, the tire is close to the fender lip. I fucked both of the fender lips up big time the first time I drove my truck.. it wasnt aired up 100% and the tire caught the fender and bent it. I've did it a few times since then, hitting bumps while turning or whatever else.. it's eating up my tires, as both front tires have slits in them. I dont understand why a 5.5" backspacing 8.5" wide wheel is so close to hitting... I've seen others with 4.75" backspacing wheels and have no problems turning. I'm not gonna fork over $400 every week to buy new front tires.. any one got any ideas? heh