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Level one is just transmission so you wont see much there.
Level two is tuning the motor and trans. That is what I keep mine on and get the best millage.
Level three is advanced motor and trans tuning but requires 93 octane gas. I have heard you get the best on three but you got to cough up the extra money and it probably ain't but 1mpg more. So in all that I would put it on level two and go at it.
Level 3 is actually for 91 octane or higher. Anything over 91, you need to bump the timing up. I run level 3 all the time and it gets aboput 11% better mileage than Level 2, so it pays for the extra cost. I enjoy the way it performs much better, plus I do a lot of highway driving and I want the torque it provides at higher speeds. Level 2 is great for around town as it provides more torque at lower rpms.
When I had my Edge on my '04 SCab 2wd 4.6L I ran it on level 2 all the time. I had 285/65r18's with some heavy chrome rims and I regularly got 19 mpg with it. It was about 70% highway and 30% city driving. On long drives I could get 20. I only put it on level 3 when gas would go down periodically and had some fun.