This is a very good question. Unfortunately, there's no indicator to tell you that you're in lean-burn mode. I wish there was.
I was thinking about this question this weekend while making a drive from Cincinnati to Indianapolis and back. I came to this conclusion (I have the 5-speed):
When you're on a flat road, at a constant cruising speed and your FCD shows 60+ mpg, you're in lean burn.
If you press the gas lightly, on the 5-speed, if you're already in lean-burn, the car will go down to about 56 mpg on the FCD, then there's a small surge as the car leaves lean-burn to respond to the throttle. The FCD will jump to 40 mpg, and any further pressure on the gas results in assist + lower FCD.
Therefore, I've come to the conclusion: if your FCD shows 60+ mpg and the engine is warm, chances are you're in lean-burn.
Mark
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