Every september we drive from Denver to Page AZ for our annual lake powell trip. I ALWAYS take off my 26s and tow with my factory 20s on. This past year I didn't have to tow the boat and I got 20mpg for the whole trip which was pretty amazing. I average high 14s normally on my stocks with my day to day driving. I average around 12 on my 26s. So IMO I would go ahead and buy the 18s.