My car has been getting crap gas mileage consistently and I am beginning to worry that it's not just from age, but from a mechanic problem. Recently, my mostly city and mostly highway mpg vary very drastically. if anyone have any clue or have a similar experience, please give advice, thank you. Here is the health history of my car.

1.5 years ago: blew a the fuel line connector at the fuel pump. brought it to a shop and got that fixed with a new plastic clip. same pump.
a year ago: gets about 24city/30highway/ 26-27 mixed
3 months ago: got it smogged and passed.
now: 20mpg city/ 23 mixed/ estimated 26 highway (never had the chance to take drive it on the highway for extended period to confirm)

driving habits:
5 days of the week is city driving (70 miles total). other 2 days are highway (45miles total)
rev to 2500rpm when taking off from a stop. (there's a lot of hills)
shift at 3500 or 4000rpm.
typically cruising at around 2000rpm most of the time, but sometimes up to 2500rpm

stock b18c1 with: (I'll try to be as detailed as i can)
hondata thermo intake manifold gasket.
greddy catch can
NGK platinum plugs
aftermarket spark plugs (cables look smaller than stock ones)
airbox intake
fidanza 7.5lb flywheel
polished intake and exhaust ports
honda crankpulley but not stock. it has a smaller alternator and power steering pulley.
mishimoto oversized all aluminum radiator with 50/50 mix coolant and wetting agent additive
mobil1 10w30 synthetic oil

stock GSR tranny with type R LSD

exhaust: custom welded 2.25" catback with ES-oval straight through muffler (2.5" in/outlet). piping is INVIDIA 2.25" with resonator

wheels:
before 3 months ago: 205/50/15 kumho SPT
since months ago: gsr blades with 205/55/15 Hankook Radial K106

O2 sensor (since 1998)
stock dual sensor gsr cat (since 1998)
stock intake manifold but port and polished internally
stock headers

maintenance:
within 2 years: new cap, rotor, fuel filter, head gasket with ARP studs, new alternator belt, timing belt, new spark plugs, new thermostat (stock setting)
tire pressure: 35psi front, 33psi rear
air filter: K&N. cleaned half a year ago. still fairly clog free.
compression check: never done this.

last tune up: haven't had one since i swapped this motor 2 years ago, i replaced most of the stuff listed above.

 

What all mods have been done in the year since your mileage was normal? Have you changed your driving habits or your locations you travel to different? When was the last time you did a tune up; plugs/wires, fuel filter, cap, rotor, tire pressure, compression check, air filter, etc etc?

 

updated the OP.

all these mods have been done 2 years ago. before the mods, i believe i had about 24 city/27 mixed. then again, i drove mostly on flat terrains with the same driving habits.