I'd be ecstatic about 18.9 mpg!
Our city mileage is usually around 16.0 using 93 octane, but did manage a little over 26 on the last tank, with a trip to Austin.
A friend of mine is a mechanic who's worked for some racing teams in the EU and he uses 87 in anything on the highway because the lower loads and constant state running produce lower loads. In the city, you have higher loads, so running lower octane is worse than if you were doing all highway driving. I've never followed his lead, I just found the logic at least partially believable.