I would imagine they would hurt it. I noticed less responsiveness with BFG A/T 315's on my old GMC, felt like I had to give more throttle to maintain speed. Sure looks nice with 315's though, and off road they get such good traction, I rarely need to lock the front hubs. Nice smooth ride too...
I think 18.2 is pretty darn good for a 4wd with 315's.
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John 3:16, John 14:6. Silver 06 Mega Cab Dually G56 2wd 3.73, Traction Bars, Gooseneck, Iggee seat covers, Tekonsha Controller, Opti Lube Summer Blend, Mopar Tailgate Spoiler, Rokktech '03 KX250/310, '01 KX500