Smaller doesnt mean cheaper anymore.... And getting a strange size 15" or 16" to make it fit right will likely be just as high (tires are cheap online...tirerack and so on) Also, if the truck was designed to have 17"+ there was some weight calculations and tread designs figured in for wear purposes.
I bet a 15" long term costs MORE w/ it being a tall side wall, thus likely a more aggressive tread pattern (All terrain) which can mean less wear, worse gas mileage and very slightly reduced cost initially.
Did I mention I used to manage tire stores....been here, done this.
12 Ram express QC, belltech 963sp and McGaughys spindles, debadged,Polk audio, smoked lights, tint, bedcover, and lots more
06 Stratos 201 Pro XL 225 Yamaha