Cold weather and short trips definitely take their toll also. Have you changed driving habits? I went from 8 to 10.5L/100kms just because I went to very short trips in city and 0 highway kms.
If you're getting 13L/100kms on the highway...in a 320...then there's something very, very wrong. BUT, don't just throw money at the problem hoping it will resolve it. If your thermostat isn't closing, that will kill fuel economy, so will a 300lbs sub/amp in the trunk. Find out what the problem is first.