[mcrews]

Assuming you have a scangauge (see link in sig) to test results.....

There are generally 4 common issues on tire/wheel:

A.) increase air pressure to 40+ psi. (see threads) It's free to do and most all see an improvement in mpg. Some go as high as 50psi. The trade-off is a harder ride. But since you are driving 'easier' it's not so noticeablr

B.) search 'taller tires':
First, decide if a 5-8% decrease in rpm at cruising speed will cause your engine to get better mpg. Drive 65, then slow down to 59 (use scangauge to test 10 mile run) Second, check for clearance issues before purchasing a taller diameter tire. Check at the strut tower and check at hard left and hard right. Next go to a site like TIRERACK.COM and see the tire measurements.
There are several members who have done great tests. here are my results:
1. Infiniti Q45: 245/45/18 stock rims went to 245/50/18
2. Kia Sportage 235/60/16 stock rims went to 235/70/16
both showed improved mpgs

C.) Reseach Low Rolling Resistance Tires LRR several members have positive results here. Capri Racer (posted above is tops on this issue)
(I did not concider LRR tires but could have)

D) Increasing WHEEL (not to be confused w/ TIRE) size.
Going to a 16 from a 15 will move the weight out 1 inch (even if it is a lighter rim). a spinning weight is 4 times the static weight. So if you havent increased the diameter of the tire over stock, you wont see a gain.
Do not increase wheel size unless you are also increasing the overall diameter of th tire. I wouldn't change rims uless they were free or you have $$$$$ to burn.