I have a 99 Civic HX M/T too. Driving at 70 mph is still lean burn as I did this Chicago to Memphis and got 51.2 mpg. But driving in Houston [with A/C] I would avg 44 mpg tanks. Driving in Chicago I would avg 28 to 33 mpg tanks.
Last year, my O2 sensor went bad. I bought a cheap $150 ebay Walker O2 sensor and they kept going bad. Then when I bought an OEM $300 O2 sensor [from Autozone] everything was fine.
Take a look at my garage do the mods I have done. This summer I hav been getting 50+ mpg. The vx trans is great because my vtec kicks in at around 73 mph. This way I stay around 70 mph and stay out of vtec.
Driving solo, I'm able to "pulse and glide" the a/c.
That is, set it on recirculate, with temp at minimum. Switch the a/c button on and off as needed to keep barely comfortable. Depending on how horrible it is outside you can run the compressor less than 50% of the time and still stay comfortable.
The usual method of setting the fan where you like it and leave it on, and use the temp slider to adjust temperature, is less efficient. The compressor run constantly that way, AND you're using engine heat to warm up the cooled air. Yeah I know, it's "waste heat", but it comes from your gasoline. The big FE hit with that method though, is running the compressor constantly.
Currently getting +/- 50 mpg in fall weather. EPA is 31/39 so not too shabby. WAI, fuel cutoff switch, full belly pan, smooth wheel covers.