[RMcNeal]

I don't believe there is a practical use difference between the 18 and 20 inch tires. My F350 SRW has 20 inch tires as part of the package ordered by the dealer before I purchased it. I have noticed that the DRW's have 19.5 inch tires in that same year. I guess some engineer decided that was better ratio for that model truck, but I'm not sure you'll know the difference either way. BTW, Ford put an 18 inch spare under my truck with the 20 inch tires, but it is taller so it will be roughly the same height as the 20's.