The criteria for good city mileage:
- light weight
- small engine
- low rolling resistance
- manual transmission
Main driving techniques:
- anticipate stops/turns to minimize braking
- engine off as much as possible
(Seen the full list yet?
100+ Hypermiling Tips)
A lean burn engine won't help much in city driving. The magic of lean burn happens at relatively low & constant load (ie: cruising on the highway), not in the accel/coast routine of sub/urban driving.
You would really benefit from a hybrid - we had another member here who recently traded his WRX for a Prius. Makes getting great MPG easier, but it's not strictly necessary. If you can't afford to
buy a hybrid, to maximize mileage you need to learn to operate your non-hybrid as if it is one! (It's what I'd probably do, simply because it's easy to find a cheap econo-beater with a manual tranny from which I could wring great MPG - cheap used hybrids are rare.)
You could get 50-60 mpg with a Geo Metro, driven properly in the city. 40ish shouldn't be too hard with the right Civic and the right technique. Avoid the models with "short" gearing (EX). They'll hurt your highway economy.
Welcome to the forum!