Originally Posted by InfiniteDice
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.
I run my tire pressures pretty high (40-42 psi cold) so I doubt it would be that, and I did the breaks and replaced my e-brake hardware and backing plate about the same time, but I'll recheck the adjustment.
However, I also had an emmissions test about 6 months ago. The car failed the first time around, then just scraped by on the second try. So I'm guessing that would suggest O2 sensor. That was also before I replaced the air filter, plugs, and fuel filter.