The extra sidewall of 37-inch radial tires mounted on 15-inch wheels helps to smooth out stiff suspensions. That's one reason we chose the Goodyear 37x12.50R15 MT/R tires and wrapped them on some used and battered 15x10 Champion beadlocks. We test fit some 40-inch tires but any amount of off-road use would have had them rubbing pretty badly even if we had cut off the fender flares. We still think the 40s would fit but you'll need to do a little more trimming than we wanted to. An axle upgrade wouldn't hurt either.