I've got 18s but I've been doing a little 'research' for my daily driving gas consumption...not really documented, sorry.
But I've noticed over the past few months that 31psi gets
me the best MPG around town (220 avg). I've gone with 30 (recommended), 31, 32, 33.
I didn't get really technical with it. Since I don't have my own air compressor I'd drive on base (1.5 miles) adjust the tires then fill up a new tank and drive til the light came on. Take it for what you want. lol It gives *me* a warm fuzzy. lol!!!
EDIT: PICS of the wheels!!! definitely a great second post....