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Since you have an automatic, I think that the easy answer is to leave it in drive.
I have a manual and I think that it's an interesting question. The conventional wisdom on the forum is to shift under 2000 rpms. I think that the real answer is not that simple. Mileage is going to have a lot to do with the weight of your left foot. If you push the pedal to the floor, you are going to go through lots of fuel regardless of the rpm that you shift. If you have a light touch, on the other hand, I don't think that you burn that much more by letting the rpms climb a bit.
For example, I often wait to shift until 3000 rpm. If I'm climbing a hill, I may let even let it run a bit more. That's not to say that I'm flooring it. On the contrary, I am giving it just enough to move with traffic. In switching between this and the shift-at-2000 strategy, I haven't seen any consistent difference in mileage (the amount of time that I spend in stop-and-go traffic has a *huge* effect, compared to this.)
My personal opinion is that it may be better for the engine long term to let the rpms beyond 2000 with some regularity. I think that the cylinder pressures are lower, the oil circulates better, and there is less change of knock than by running at lower rpms where the engine may lug.
I'll throw out the caveat here that I am *not* an expert in this. I would welcome discussion on whether my thinking is either in line, or completely wrong - or somewhere in between.
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