I am about to order an F350 with the 4WD package that comes with 17" cast wheels. For an extra $900 I can get the cast 18" wheel/tire set. For off-road use here in the Southwest with plenty of rocks and sand I wonder if I should consider the standard 17" and possibly put forged or steel 18" rims later or would the factory 18" be sufficient? I really don't want to lift it for larger wheels. Any major reason not to go with the factory 18"? I am not planning on any extreme jeep trails.

 

All other things being equal, taller tire sidewalls are better for off roading. Most find that anything less than 8 or 9" of sidewall is getting pretty compromised, with 10" or more even better for serious off roading.

So unless you're going to go with 35" or taller tires, I would would recommend the 17's.

 

Thanks, so you don't think the extra diameter would help much in soft sand or improve the ride much on rocky roads? Both diameters are 70 series.

 

If you're going to be on sand, I'd go with the 17" wheels. If you're going to air them down, drop the pressure to whatever, 20lbs or so, the smaller wheel will give you more sidewall (for the same overall height. The sidewall will allow the tire to "flatten" out more and you'll have more "floatation". So, for most airing down situations, you'll get more floatation with the smaller wheel.

The thing though is you said they'd both be 70 series, so you're talking about metric sizes. 70 means 70% of the width. So, 70 is 70 is 70. There isn't an advantage to either wheel in that case, assuming the width of the tires are the same. If the width (265 for example) is the same then you're getting the same aspect ratio (70 for example) and that means the sidewall size will be the same. The difference between 17 and 18 inch wheels will be overall tire size though, because the wheel size then determines the height change. For example: 285/70/17 and 285/70/18 are the same width and the same sidewall, but the 18" wheel means the 18" version of the same tire is taller.

If you convert it (and I didn't look up the exact measurements), you'd have a <11.5" wide tire, 70% of that (the 70-series part of the equation) being ~8" sidewalls (remember you have both sides of the wheel with a sidewall so 16" total) and then you measure the size of the wheel. A 285/70/17 would be 8*2+17" or a total height of 33. A 18" wheel would be the same, 8*2+18 or 34" height, with both having about 11.5 width.

 

If you like to spend more money on the tire itself the 18'' tires is for you and the choices are not as great as the 17''. On my '05' truck I ordered the 18" tires and now I wish I would have ordered the 17". Go to tirerack.com and just do a search of tire and $ amounts.