I know this isn't for everybody, but thought I would post it up here. We are one of only two places in the country (that I know of) who are now set up to swap a 5.7, 6.1 or 6.4 liter engine in 2007+ Wranglers. I just got back from driving a few thousand mile wheeling and road trip in my JK with the 6.1, it's the perfect mod.
We have three 2008 model 5.7L MDS Hemis in stock. The 5.7 should get about 16-20 mpg regardless of tire size.
I am willing to give a club discount on these swaps to club members, just call and ask for Kevin. 417-862-4946.