That's an admirable goal and a fine hobby... but be aware that for the most part, it's difficult to get a real return on your investment when you're modding for mpg. This can be expensive, and the savings can be modest.
350 miles on ten gallons, at $3.50 per gallon, means you're spending ten cents a mile. If you can increase that to 40 mpg, that's 8.75 cents a mile.
Say you drive 20,000 miles per year. That's a savings of 250 bucks per year. If the car lasts you 5 years, a total savings of $1250. But how much did it cost you to get that extra 5 mpg?
The biggest (and cheapest) mod is your driving habits. You're getting 22 mpg on E85. That implies that you're driving like I do, hammering it down on the on ramps, going 10mph or more over the speed limit most of the time. As an experiment, last summer, I went through an entire tank driving around the Dallas/Ft.Worth area like I normally do, and then another tank driving reasonably; I accelerated slowly, kept it to barely over the limit, was more careful to begin slowing well before stops. The difference was astoniching, I got 22 mpg on the first tank and 33 mpg on the second, and that's without using any hypermiling techniques or doing anything that would annoy people behind me. Cost? Free.
Eco tune, by the way, hasn't been proven to help with mpg at all. Driving the same way, I, and others I've seen report on this, get the same mpg whether on eco or performance. I admit that if I really hammer it, I can spend more while on performance... because eco limits the amount of power (and thus fuel consumption) I top out at. And I am more likely to drive gently in eco than in performance. But if I behave the same way, then there's no appreciable difference.
Another inexpensive and productive thing to do is minor maintenance. Check your spark plugs, make sure they are all gapped the same, and consensus seems to be a 0.028 gap is best... but you might play with that to see if a little smaller or larger helps. Mostly, though, an uneven gap is the enemy both for performance and for mpg. Inflate your tires as high as you are comfortable with, remembering that overinflating can cost you traction. Keeping the car clean improves aerodynamics a little... which means a little cheap mpg improvement, especially at highway speeds.
After that, though, things start to get expensive. New lower resistance tires, custom building body parts, replacing mirrors with cameras... I can imagine spending $1000s on something that only nets you 1 mpg savings, so it would take you decades for it to pay for itself.
