Funny, my average fuel economy gauge resets on it's own every fill-up.
I drive 110km one-way to work, 99% highway. I have 6600km on the car, and am averaging 7l/100km. Best is 6.4, worst is 8.1. The 6.4 number is an average over 450km, the 8.1 over 225km. It's been trending downwards, but has probably plateaued. These are calculated numbers (Android app called My Cars), the rating from the cluster is generally .2-.5l/100km optimistic.
I generally drive around 120km/h(limit is 100), if I'm being particularly patient I drive around 110km/h(probably when I hit 6.4l/100km). There are occasional (fuel sucking) bursts up to perhaps 140km/h (Interesting tidbit, getting caught driving 50km/h over the limit in Ontario, Canada will net you a 7-day roadside (i.e. - immediate, they tow the car) vehicle impoundment and license suspension).
On the highway, your gas mileage is going to be mostly effected by your speed - wind resistance increases at the square of speed, so the faster you go, the harder it is to punch a hole in the air.
In town, the best thing to do is watch traffic ahead and try and engage the Deceleration Fuel Cut-Off (DFCO) as much as possible. If the car is rolling at a sufficient speed in gear, and the throttle is closed, the injectors don't fire. So if you're rolling down a hill at a stop light, just close the throttle completely and you won't be using any gas.
I've definitely noticed the motor doesn't seem as reluctant to revving to higher RPM's as the kilometers have accumulated.
Thanks,
Brian