I just bought a '92 Civic VX (California spec) from a friend. It has ~300k miles on original engine. My first tank of gas, 22 miles per weekday commuting on the highway, plus some city driving, not trying to nurse it but not being a jackrabbit, using A/C as little as possible, I got 37.33 mpg. This seems a bit poor to me, I expected mid 40s around town.
It has some known issues: ECU Code 12 (EGR) and (just appeared) Code 48 (LAF) so I need to address those. I plan to pull, inspect, and clean the EGR. As for the LAF (Lean Air Fuel sensor), that seems to warrant more careful debugging, any counseling?
Also, the speedometer has lost its mind. It seems to do whatever it wants to randomly, including spinning clockwise all the way until it gets stopped by the zero stop, then sits there on the bottom (wrong) side of the stop. Other times it seems to work, but will get stuck at some speed like 45 or 100 (no I don't drive 100!) for a long time. Since there is no ECU indicating a VSS problem, so I think the speed head it kaput. Any comments welcome.
I suspect the LAF is affecting your fuel economy and the issue with the speedometer affects the odometer too, resulting in bad data for your fuel economy calculation. Can't say I know how to fix them, but hopefully someone else here can help with that part...