'98 Civic HX poor fuel economy

[cems70]

Hello all,

The fuel economy on my wife's '98 federal HX with 124,000 miles on it is worse by 5% - 11% versus the previous two tanks. I'm sure some of it is the result of colder weather, but not all of it. Her driving hasn't changed at all (daily 85 miles roundtrip, 98% highway with the possibility of 4 traffic signals along the way, some rush hour traffic periodically, 70 mph avg speed). Last year at this time she was getting 45 - 47 mpg. This year she's getting 39 - 44 mpg. The only thing that's changed since last year is a set of new tires (Nokian i3 LRR). Perhaps some of the difference in FE versus last year is due to a larger tire diameter now registering fewer miles. But the tires do coast well, so I'm certain they are LRR.

In the past 1.5 years, the CEL has come on periodically, and the code is for the primary 02 sensor. Each time I disconnect the negative battery cable and reconnect it to reset the ECU. There has never been a change in FE during these periodic CEL episodes. During this latest drop in FE, the CEL has not come on. I did a tuneup at 109,000 miles (plugs, cap, rotor, air filter, valve adjustment). The engine appears to be running well as always...no symptoms of any problems.

Does anyone have any suggestions about the cause of the poor FE? If it's the O2 sensor, is there a test for it? And if it's bad, are there any recommended aftermarket O2 sensors that are cheaper than the $300+ Honda O2 sensor?

Many thanks for any suggestions.