It also depends on driving conditions. If you drive in a lot of stop and go traffic, expect to get poor gas mileage in the Z. If you're on the interstate getting those kinds of numbers, then something isn't right.
I get around 18.5 average but I deal with a lot of traffic every day. Even on the interstate. When I did a little longer interstate trip a couple of weeks ago, I was closer to 22 or so.
I love
the Fuelly website for keeping track of all my fuel usage, prices and mileage. It can really help you with tracking your results over longer periods of time so you can spot trends on your fuel economy. Plus, it gives you a nice record of how many miles you're really driving during any given time period.