I'm thinking about getting a 2wd F250 CC SB V10 with 4.30gr. Just wondering if the 18" wheels will add much to the towing capacity, when pulling a 5th Wheel Trailer. The 17" will probably have a better selection of replacement tires, but that might change by the time I get the truck and then need to get replacement tires.

 

It will probably subtract from your tow rating. The larger the tire/wheel combo, the lower the tow rating. My F150 tow rating dropped 500lbs due to the 18" tire/wheel combo from the 17" tire/wheel combo. Reason is the larger the tire/wheel combo, the higher the final gear ratio. A 3.55 ratio has a lower tow rating than a 3.73.

 

Also check the tire sidewall for tire capacity, this will be a determining factor.

 

Dave, the 17's (that are stock) have a load rating of 121(3,197 lbs per tire). The 18's have a load rating of 125 (3,640 lbs per tire). The diameter of the two tires are close to the same. I have 18's on my truck and 70 mph equals about 1900 rpm. With the 4.30's and the V-10 you should have no problem towing with either tire. The max loaded 5-W weight rating for the combination you mentioned is 16,000 lbs. and a GCWR of 22,500 lbs.

Yes, I have to think (hope) that there will be a better selection than the Conti's that are standard by the time I need new tires.

 

If you want the best towing performance, get the stock 17" 265/70R17 tires. They are 31.6" in diameter. For carying capacity, the 18" in either the 275/65R18 or 275/70R18 would be better. The 65 series are 32.07" in diameter and the 70's are 33.16". Obviously the 31.6" tires will tow better than the 33.16" tires.

If you do go with the 17's you can upgrade right now to a 285/70R17 (3195#'s) tire and be in at 32.7" diameter. Lots of choices in that size.