Thank you all for the responses and feedback. Agreed, I am not sure I'd see much on the mpg gains, but as Frank indicated, the ride should improve.
@oilpan, I found a set on Craigslist, with tires for $400, but they disappeared the day after I emailed the seller.
I'll be searching regularly though. Several guys on DodgeTalk that drag race their trucks (yes they do) go to 17" steel wheels, 'cuz they are lighter than the OEM alloy 20s.
@pryan, good point about the payback vs tire replacement.
@slowmover, I'll be looking at the 2014 Chebbies. I'm curious now...