I'd like to build city-scale (6 studs wide) motorized vehicles with PF remote control. There are already several suggestions on how to motorize wheels. The next challenge is steering mechanism that can be connected to motor (with gears or by other means). Sure, solution with 2 rotation axes is preferred, but rigid axle turning at center is OK too.
It's difficult, but not impossible. A rigid axle turning at the centre would be mechanically simpler. As my crude drawing illustrates; I would place a 24-tooth gear over the 2x4 wheel axle. A 8-tooth gear is optional but will provide finer control over the steering. The two teeth gears can also be replaced with a pulley gear.
This technique however will not work with current Lego CITY vehicals unless the wheel arch is removed or replaced.