The best mileage I've gotten on my GT was during the breakin period. I went on an all highway trip and kept the cruise control at 55MPH the whole way (thought I was going to get rear-ended numerous times!). That got me 25 - 27 MPH. Since then (post break-in period
) I've struggled to get 20MPH doing mixed city / highway and short distance (<10 miles) driving.