2000 Volvo V40 Poor gas mileage

My car is only getting 16 mpg city driving. my driving is mostly city(short trip) and perhaps once a week freeway.

I live in the San Francisco bay area and moved from from LA county where at least 1/2 of my driving was city. Shortly after I moved i changed the spark plugs and I don't know whether or not this was factor but it seemed after that, was when my mileage worsened. The car except for that runs flawlessly

Thanks