Heavy tune up/ re build

[superbraut]

Okay, so i have a 1990 240sx with 145k on the ka24e motor. Im getting pretty bad gas mileage right now (15-20mpg), and just now my engine light came on. I was thinking about doing a swap, but i dont deal with smog and mods.

I want to do a Heavy tune up/ rebuild to get the most mpg out of my car. I recently bought the car used, and the previous owner said he did all the work himself fluid changes, but thats about it. What are some advice i can get on rebuilding my ka-e to get better mpg. I specifically want engine rebuild advice, all other mods i already have in mind. I plan on doing this within a month or so, by then i can potentially have 2.5k.

[SullivanRacing06]

piston rings and new main n rod bearings, basic rebuild.