I have another theory. My thermostat was stuck open, so cruising in the cooler weather would sometimes cool the engine down to 60*. Since the engine management is ultimately triggered by the coolant temp sensor, this would cause the engine to operate in open loop mode without the O2 sensor on the highway. So considering that, do you think that going without the O2 would yeild higher efficiency?

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

I should add that I did fix the thermostat and now it is rock solid at 87* on the highway

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

An O2 sensor is usually the king pin of fuel efficiency. if it goes bad or the loop is broken, it'll cut the mileage by half.

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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FACEBOOK: PANZER450

Thats what I thought too, but I'm still learning about CIS so I figured I'd throw it out there.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

Have you ever verified your mixture?