[azraelswrd]

Pure city driving?

+Idling
+stopping at stops/lights
+accelerating from stops/lights
+cold engine status
+low tire pressure
+excess dead weight

...are things you should address or fix. At lower speeds, tires, engine condition and driving styles have the most impact on fuel economy. If you don't already, get into the habit of anticipating any and all stops and either gear-coasting or neutral-coasting to where you need to be. Also consider a route that is more conducive to fuel sipping. I used to take the shortest distance but the idle times wasted more gas.

50mph sounds rather "hot" for pure city driving. I rarely breech 45mph on the straightaways in my routes and even then that is too fast at times especially when traffic gets thicker. Consider slowing down a little more, but offhand, 40mpg just with 100% city driving isn't really that bad. I get about 38-42mpg in city (depending on traffic flow).

Feedback monitoring device like the MPGuino or vacuum tube should also help.