The EPA estimates for my Bimmer is 20city/29hwy and in real world driving, it's usually lower overall around 18city/24hwy. Here the secret:
Some BMWs come with a mpg gauge that's actually a vacuum gauge linked to the manifold. The higher the manifold vacuum, the better the mpg.
And now the kicker: I don't set the cruise control and forget it on the hiway. Instead, I jockey the plus/minus rocker switch to keep the mpg/vacuum gauge over 30. That's right, when I'm going downhill, I accelerate. When I'm going uphill, I decelerate. I travel between 65mph and 75mph and on long flat stretches of road, I average 70mph and ~29-32 mpg. When traveling at or around 65-70 and I see a hill coming up ahead, I start clicking, slowly, the minus button and move over to the right lanes. I can time it to slow down to 55-60mph, all the while keeping the mpg gauge near or over 30. Right before cresting the hill I stop clicking the minus and start clicking the plus going downhill until I reach a cruising 70 or if I can do it safely, 75-80mph, again, keeping the mpg gauge near or above 30(usually 40-50, downhill). My instrument cluster has a digital avg mpg and it hasn't gone below 30mpg. On some days with a tailwind, I get 38 mpg going, and 29 mpg returning. That still averages out 30+ mpg. This technique isn't for the novice. I can see the gauge from my peripheral vision and keep my eyes on the hwy. If you're not used to it, you can get into an accident. I suggest new users have a copilot/passenger keep an eye on the gauge while the driver does the driving. The copilot can inform the driver when the gauge dips below the desired mpg, while the driver adjusts/jockeys the cruise control buttons. Warning, avoid this technique in heavy traffic. If you drive daily in heavy, stop and go traffic, or a lot of city driving, where avg speeds are often 0-30mph, get a Prius, they're designed for that.
If you don't mind a harsher ride add 2-4 lbs of air to all of your tires.
If you can, try to get stiffer sidewall tires, there's less rolling resistance.
If you can leave earlier for work/go home later you'll avoid traffic, that means less braking.
If you can, keep your windows up at high speeds, the wind drag is less.
Remember, all cars can use regular unleaded, if yours starts to knock, then go to the next grade up, or mid-grade. Premium is for older cars or high-compression engines.
Most of all, avoid the gas pump island that also puts out diesel, there's usually some cross-contamination that'll foul your plugs/injectors.