O2 sensor does next to nothing on long highway trips. I know this because I ran no o2 sensor for years and still had great gas mileage in my old 325e.
In town having no O2 sensor, or a malfunctioning one wastes more gas.
I'd say have the emissions tested, this should point out a problem if there is one with the engine/fuel-air mix.
On another note, when you stop driving, quickly walk around the car touching all the rims. If one rim seems hotter than the others you could have a siezed caliper. Also a dragging parking brake can cause some drag. I would also check your tire pressures.
Performance one step at a time.
Last edited by InfiniteDice; 08-18-2011 at 09:27 PM.