98 Civic DX bad mpg
 

Got this car from a friend and I replaced the head gasket because I mistakenly thought that was the whole issue. Turns out it was the distributor so that was replaced with a new (expensive) unit as well. PCV, spark plugs, fuel filter, oil in trans and engine changed, new radiator/thermostat, previous owner installed some k&n cone filter and piping but left the stock box around the throttle body.

CEL code for secondary O2 sensor so that was changed and there are no other CEL lights. No other issues with the car other than it doesn't get great gas mileage. I pump 89 octane from shell/sunoco and I'm down to right about 1/4 tank maybe less with 170 miles on the trip meter.

What else can I do here? Seems like I did all the maintenance I could do. Car has 169k on the clock but still runs great. Got the car at 150k

Did you set the timing after changing the distributor?

Also the dash fuel gauge is not linear. You need to fill up again and see how many miles driven vs. gallons purchased. Ideally keep records and average it over several tanks.