One more thing, o2 sensors are notorious for causing poor gas mileage.

 

Long shot here, but most people have said what I would have said... It could possibly be that your fuel pump is dying. That would explain where and why there is that buzzing sound. This could also mean that the engine is running rough because the pump could be pumping not enough gas into the engine at higher speeds, disrupting the fuel to air ratio and therby murdering your fuel economy.

 

Loss of fuel pressure or volume could cause an engine to run lean not rich. OP stated there was a smell of fuel from the exhaust. To much or unburnt fuel is getting into the exhaust.

Stick with checking O2 sensors, injectors, coils or plugs. But I suspect that the OP has resolved the issue since this thread is two months old.