I'm always watching mine in the highway. If I keep it less than 75mph I'll get 23-25 mpg depending on wind direction, temp, etc. - lots of variables as stated by others. If i go closer to 80 mph I'll get 22-23 mpg.
Here's an interesting situation I noticed the other day, I was playing around on the on-ramp and hit 100 mph, then coasted down to 75 mph as I merged on to the hwy. I also hit the reset on the fuel economy reading as I was in decel mode. As I expected, it showed 99 mpg and gradually dropped as I neared 75 mph. Here's the funny part, it read 28 average mpg for about the next 10 miles. Then some guy in a new Camero SS wanted to take me on, so I had to give him the "spanking" he deserved - my mpg dropped to 12 as we hit 140 mph.
BTW - I have 3.55 gears in my 12 GT w/brembo brakes.