Just to add a data point, tracked my mileage on a 60 mile round trip commute yesterday. My car is a 135is 6MT convertible, AC was not in use, top was down for return trip, route is 65% highway with modestly rolling country two-lane roads for the rest. Averaged 30.1 while setting cruise at 70 or so on the highway. Was modest on the throttle, but not babying it.
Overall, quite pleased to know that on interstate drives, reasonable fuel economy is possible, especially for the "is" model with the extra weight and drag of a vert with only 650 miles.
Note, with city driving and some sportier driving last night, tank average is down to 25 today. Overall, I'm expecting 22-24 for mixed driving once I can finally hammer the car after break-in is complete.