I tend to put on mostly highway miles, 100-160 miles at a time, and my mileage is still only 40-44mpg at it's very best, I know my 92 civic vx can do better! but I'm running out of ideas of things to check/tune/replace.
New front bumper (old one was missing a chunk)
New throttle body.
New o2 sensor.
New 165/70 tires set at 44psi (max rated) front, 35 psi rear.
Synthetic 0w-30 engine oil, 10w30 synthetic motorcycle oil in tranny (had 0w 30 and a bearing was making noise)
a/c removed, grill to be blocked off as soon as the part shows up.
New cap, rotor, wires, ngk zfr5f-11 plugs.
New pcv valve, radiator cap, thermostat.
New larger oil filter to give more filter media/less restricion.
Removed and cleaned fuel injectors.
Replaced exaust and muffler with origonal fit aftermarket parts.
Replaced timing belt and water pump.
I normaly use 92-93 octane 100% gasoline.
I just ordered new oem suspension bushings as my front wheels have just a little play linked to the lower controle arm bushings.
I also ordered oem thermostat, I don't really trust the generic and I haven't been able to find a 192F thermostat, and don't want my fan coming on if it's not needed, and haven't been able to find proof/numbers that hotter is better, just theory so far.
I carry 200+ pounds of work related "stuff" with me alot of the time, but I try to go thru and remove extra unneeded weight when possible.
my compression is 180-185 accrose all cylenders (ideal is 185, 120psi is lowest alowable spec) and the engine seems to run smooth, you wouldn't know it has 220,000 miles on it.
I've thought about a belly pan, and I might make one, but as it is the front bumper's farring extends back nearly to the front axle.
So what can I do? I've been working on pushing this car for better mpg for year that I've owned it now and it's stayed at 40mpg for most of that year, creaping up a little but hardly noticable, it's not even reaching the epa mpg.
i think it would benefit ever so slightly from a pcv catch can, just to help out with the octane of the air/fuel mixture. also cleaning all that gunk out of the intake manifold from all those miles will help raise the octane of the air/fuel mixture. those two go hand in hand
you already mentioned the belly pan which will definitely help. how is the alignment?
also i noticed two filters werent on your list: air filter and fuel filter, with 220k miles each could hinder the mpg. have they been replaced recently?
don't waste your time or time will waste you
I'm curious to know why you are using 92-93 octane. That engine is made for 87, so unless you are living 4000 feet below sea level I don't know why you'd need such high octane gas... unles you enjoy spending money.
I second jared's recommendation to get a new air filter and a new fuel filter. Don't forget to chagne the brass washers when you change that fuel filter. Trust me on that one.
Was it you that had the california VX, or was that someone else? If it's you, you really need to look into converting it to non-CA VX. It will take an extra wire and a new ECU. With that modification alone you'll break 50mpg (lean burn mode). If I'm thinking of someone else, please ignore me
At highway speeds your biggest mpg killers are due to aerodynamics. Work on that belly pan, and start brain-storming about rear wheel skirts. Maybe even getting some flat hubcaps would be a good idea.