I don't drive a Civic, but I'll tell you the single biggest factor that influences my short-trip fuel economy: initial engine temperature. Making a short trip on a warm engine can make a difference in fuel economy approaching 25% depending on just how short that trip is. Take this example with a grain of salt:

The trip to the local grocery store is 4 or 5 miles, down hill the entire time. I just have to get over one "hump" and then gravity does 90% of the work until I get into about a half mile of frequent traffic lights. On a cold engine I might manage 38-42mpg. (Did I mention that this is down hill the whole way?) But yesterday I did the same trip and broke 53mpg because I did it right after work, with the car fully warmed up. (And before anybody cries foul, the best return trip I've seen was 28 or 29mpg.)

So what I'm saying is this: if you drove to the gas station, filled up, and then made your trip to work, you may have reaped the benefit of not having to factor in that cold-start penalty that comes from running rich and idling high until the coolant temp gets into the "normal" zone.