Most of my driving is commuting back and forth to work on the highway at about 100-110 km/hr. Some limited city driving. As I said, with the same commute, same driving style and just the replacement of a filter and plgs (I didn't remove the battery cables for that, so no ECU reset), I jumped from high 300's to mid 500 km per fill up (I do calculate by odometer and litres per fill so that I do calculate this more precisely). After a few months, mileage started to go down again.

I've tried varying my driving style, changing where I fuel up, and two treatment of GM fuel injection cleaner. Noting has significantly changed my fuel ecomony.