I do a lot of city driving (I hate it), but whenever I'm on the freeway in fifth at 2900-3200RPM my gauge never moves. Occasionally it will go up a few miles, because I'm getting better mileage.
I just don't think you'll see a huge difference cruising at 2100RPM, except maybe some premature engine wear from things building up in the engine. There was a thread a while back, that explained the going to 5000RPM is good for the car because it cleans the engine out.
Plus, going slow is just no fun!
