I have an '02 wrx wagon with 254k + miles. When I bought it, with 227k the only modification to the car was a K&N drop-in filter. Drove it home 1,910 miles, and it averaged about 26 mpg, averaging probably 70-75 mph.
Since then I've done:
aem cold air
godspeed turbo inlet
PnP ceramic coated stock turbo
GTspec EL headers
38mm tial mvs ewg w/ dump tube
3" HKS turbo-back
ebay FMIC with stock bpv
1 stage colder NGK plugs
sti pinks (cleaned and tested prior to install)
13lb act street flywheel
gimmick lightweight front pulley
uncle scottys tranny cocktail
tuned by sti mikey via romraider
and I'm sure there are some other things I missed.
However, I currently get about 24 mpg driving around town (I let the car warm up EVERY time) and the best I've gotten long distance is not quite 31 mpg.
I also plan on removing the stock hoodscoop and putting in a reverse flow scoop, or just a black piece of sheet metal, and have heard I can pick about about 1 mpg with that. So, we'll see what happens!
I'm sure not complaining about the mileage though!