Hey, FWIW... you're best (and cheapest) solution is to drop back to 60 mph. Just chill out in the right lane a little bit slower than everyone else and you'll see that mileage come right back up. With 3.73s and my 5.4 I get about 18 highway at 70. When I increase to 75 it drops down to 16... and at 55 I get slightly under 20 (like 19.5-19.9 range). My speed makes a huge impact on mileage so I choose to just drive a little slower.

 

It's more your rpm's Going form 55-75 is like 500 rpm's diff which if you go by what I said above is like a mpg per 100 rpm's...

 

I agree, slowing down to 60 mph on the highway would be the easiest/cheapest solution; however, I'm in construction and am on the road a lot, so since I already waste enough time driving, going slower just isn't an option for me, as stupid/stubborn as that may be.

At this point, if I keep the current setup, I need to spend $400 on a tuner to adjust the speedo and not run up the mileage unnecessarily (affects resale and warranty, which is serious considering I paid for the extended warranty). If I drop gearing, I spend more than that, but don't have to buy a tuner and get the benefit of getting mileage back. So, I think that's the route I'm eventually going to go, just trying to decide exactly which route to take out of the 3 options I see (go down to 3.73 and keep the wheels, giving it effectively a 3.49 ratio, go down to 3.55 and keep the wheels, taking it back to basically as it was stock, or go back to 3.31 and go back to stock wheels).

Thanks for all the input, all comments helped me see the whole picture, the options, and the pros/cons.