In my stock 318i with the M42 1.8L engine. With one person driving in the car 90% of the time...On a full 17.5 gallon tank I get about 300 miles of mixed city and highway. I also use 93 or higher octane. On straight highway I've gotten 350+ miles on a full tank.
If gas is your concern and not speed or racing then a 318i is a perfect pick especially if you do alot of basic everyday driving around...I've put almost 20,000 miles on my car since I've bought it last June 2004. Some weeks I've driven on average 700 to 900 miles....from just doing basic stuff and errands.