Need some help here guys!
Got a 99 Civic, 1.6L SOC. I got it at 107,000 miles, got new tires, did a tuneup, oil change, the usuals. Well my friend get 400miles a tank (exact same car 99 Civic EX 1.6l SOHC), and I only get at very most 300. I have done the following...
1. Checked timing
2. Full tuneup, cap, rotor, wires, air filter
3. Replace upstream oxygen sensor (replaced with universal, (I'm not paying the $140 difference for the oem connector)
4. Replace fuel filter, have yet to check pressure, still deciding how to make an adapter
5. TCC is functioning properly, compared rpms at 60 with my friends, very close to the same
6. Timinjg belt was changed 2000 miles ago, as well as the water pump (Didnt affect fuel economy).
I'm at a loos here guys! Please advise me of any ideas you guys might have. I'm sure there some things I've checked and just forgot to mention!

 

Update:

I checked the fuel pressure today, 40psi running after the fuel pump...

I am really at a loss here guys, please give me any advise!! I do nearly the same highway driving as my friend, so thats not it either, my tires are at 33psi and have less then 5000 miles on them...

 

It's difficult to make a real mileage comparison with your buddy when you measure gas usage by the tank. The dash gas gauge is not very accurate and may vary between Civics. What precise mileage (miles/gallon of gas) do you and your buddy get? It's difficult to advise you without these numbers.

Nonetheless, some other items to consider:

-Did you check the ignition timing with the service connector jumped, the engine warm, and the engine idling to spec?

-Is the fuel pressure regulator working properly?

-Are your brakes dragging?

-Do you drive aggressively compared to your buddy?

-Is the engine idling properly? Is the PCV valve/breather system clogged?

-Do you have an automatic transmission whereas your buddy has a manual?

-Do you have an exhaust leak in the exhaust manifold or pipe upstream of the O2 sensor?