BFG A/T KO 285/75-16 spec a 7.5-9" wide wheel.
BFG A/T KO 315/70-17 spec a 8.5-11" wide wheel.
You want to go .5" narrower than that spec on the 285s (7" wheel) and in the next post Agile Offroad says they've been going .5" narrower on the 315s (8" wheel) for years. Just the facts I reckon. Success breeds success so they say. A 34.4" tire <> a 35" tire btw and in spite of being a "315" those only have a 10.4" tread width so I bet a narrower wheels fits great, as it does in a 265, which that tire really is. 315/25.49= those tires should be 12.35" wide, and they are, but only in section, not tread. Lame.
B.F. Goodrich very much expects and designs their All-Terrain, Mud-Terrain, etc tires to be used off-road. Their specifications are just that, specified parameters under which their products are designed to perform. If you mount them on wider or narrower wheels or exceed their pressure or load ratings you do so at your own peril and should expect performance to diminish. You buy tires to provide traction (a combined result of weight and contact patch equalling ground pressure coupled with friction) and to carry a load on a cushion of air, all of which depend on their shape when mounted and the air pressure within them so wheel dimension and inflation pressure are the only variables you can control within a single tire size and tread design, and thus both are clearly and specifically denoted by the manufacturers. Nobody knows more about tires than the folks making them. As for 4wd vans, Ford hired Quigley to build them and sold them through their own dealerships, even offering warranties on the modified vehicles (think Roush Mustang, Centurion F-Series, etc) so yes they do anticipate them being used off-road (ever notice the little Driving Off-Road pamphlets in Owner's Manual packets?) and neither Ford nor B.F. Goodrich wants any damage claims they can be liable for so they specify the parameters under which their products can be safely used. If you're interested in using your van unsafely just ask, I have lots of bad ideas. As for wheel widths, you want the tire tread to be flat across the contact patch for best traction, lowest heat, and maximum tread life on hard surfaces then air down for more flotation and improved tire flex off road, all of which are benefited by a wheel that keeps the sidewalls basically vertical at street pressure, which is the manufacturer's intent so their suggested wheel width is your ideal. A wheel too wide requires air pressure too high for a flat contact patch due to the tire sidewalls being extended outward and will either wear the tread's edges prematurely or will shorten tire life due to chronic overinflation to counteract the tread pucker. A wheel too narrow conversely requires air pressure be excessively low due to the tread bulging in its center and will either wear the tread center prematurely or will shorten tire life due to chronic underinflation to counteract the center tread bulge. Inflation pressures can also be too high or low for a given load, even on the same and correct wheels. Chronic underinflation was the root cause of the Ford Explorer/Firestone debacle but not because of wheel width, rather the light load of the vehicle on (relatively) large, stiff (C range) tires that were underinflated, thereby softened to provide a smoother ride caused overheating and rampant tread separation. We're talking about tires rated for half-ton trucks (F150) being used on what were basically 5 passenger Rangers. OF COURSE the tires were stiff. Load charts are available from tire mfgs that specify inflation pressures per load and some even vary by ambient temperature. I said all that to say this: The tire's "Measured Rim Width" is the wheel width it was mounted on when all other measurements were taken and tests performed. It's my personal suggestion to use a rim of that width if possible and if not, as close as practical to it while by all means staying within the range of acceptable widths. Inflate to suit the terrain and the load. Read each RELEVANT mfg specs. Let BFG pick ur wheels, Ford pick your motor oil, Warn pick your winch mtg bolts, etc. (It was Ford who lowered the inflation specs. Folks following Firestone's specs had a bouncy ride, but no tire explosions) Be smart. Be safe. Have fun.
