On the final 45 miles of a highway trip I filled up the tank and reset the trip computer's average MPG and it was reading 25.2 when I reached my destination just off the interstate. I specifically tried to get good fuel economy on that leg just to see how much I could eek out if I really needed to. I used cruise control most of the way set it between 71-73 mph indicated speed.
The problem is I don't think I could hypermile a whole tank considering the addictive nature of this engine's power and responsiveness. You really don't buy this car for the fuel economy, but it certainly seems you can easily get well over 20 MPG with these cars! I'm kind of pissed off that we have to pay the gas guzzler tax since if it were EPA rated at 23 MPG highway (certainly attainable with the M3) there would be no penalty.