I'm pretty surprised to see these numbers, with stock rims and Michelin tires (Primacy MXV4) I believe, the best I ever got was 910km(570 miles) on a tank. Around 50km in city and the rest were all highway km's on a road trip. The fuel tank is around 62 liters..Gives about 6.8l/100km almost all highway. Converting it to MPG that is around 35MPG. I'm surprised because my car has 310k (almost 200k miles).
I since then put 17" 225/45/ (Michelin Pilot Sport front, Bridgestone Potenza's in rear), and have went on a similar road trip, averaged 7.6l/100km (31MPG) however, I also put a new cat, new exhaust, and cold air intake. (not sure if it would change results much)
99 4 cylinder, 5 speed