Where on the fuel map are you running lean? Your fuel trims are likely postive % indicating that fuel is being added to the factory calibration at some level of load. If the trims are positive under low or no load (idle) that the likely solution is that you have a vacuum leak. Under high load (for example, highway driving) your throttle plate is open, which is a vacuum leak by itself. A minor vacuum leak will not contribute to increasing the fuel trims at high load because the throttle plate itself is open. You will see the fuel trims level off at this high load. You can observe that with a live data scanner. I suggest checking your fuel trims, and checking for a vacuum leak with a nice stethescope.

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