I drive mostly city with one or two 50 mile (round trip) highway drives a week. I've let my average fuel consumption run for the last 4,000 miles. I'm at 24.2 MPG. I know it's a solid average, because it doesn't move from 24.2, even after one of the 50 mile road trips.
One thing I've noticed with the 135i (and all turbo cars) is that fuel economy is affected significantly
by driving style. With a naturally aspirated engine, the intake always operates in a vacuum, so the volume of fuel-air mixture circulating through the engine can only vary up to a maximum that the engine design can flow. With a turbocharged car, you have positive intake pressure literally forcing more fuel-air mixture in to the engine. This means you can either sip fuel while operating at partial throttle, or you can GULP fuel under boost.
Another significant factor is the way fuel-air mixture is controlled in a turbocharged car. Ever notice that a turbo car will put out a puff of smoke when you floor it? This happens even to brand new turbo cars. This is the engine management system forcing a rich mixture in preparation for the onset of high intake charge temperatures and rapidly changing combustion chamber conditions. All engines, even N/A ones, richen the mixture on acceleration, but this effect is even more significant in turbocharged engines.
It takes a lot of discipline to get good fuel economy with the N54/N55. You have to go easy on the throttle. Accelerate smoothly under light throttle and try to avoid "stabbing" the gas. You'll get much better mileage. Then again, I'm assuming you bought a 135i for the same reasons I did. I want to feel the go-go! I've been through two sets of rear tires on a 36 month lease, and my turn-in isn't until mid-July