Mileage can vary by up to 10% depending on the season. Some states require that fuel contain a 10% ethanol mixture during the winter months. In Minnesota it's 10% ethanol fuel year round, I don't know what it's like in your part of the world. The difference between 19 and 22 mpg is about 15%. This may or may not be significant. Have you tried averaging your mileage over a month to see if these figures hold? I would try and find out if the fuel was switched for the winter, if it was, your economy problems might be solved come spring.