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!

 

I don't know what to tell you. Are they both the same transmission? Manual and Automatic could make a difference. Also what kind of driver are you? Do you find yourself in the High RPM's a lot? Does your friend do more highway driving to than you? Do you have rotational Tires if so are they rotating the right way? (sounds silly but its true.) Last but not least maybe one of you has California emissions and the other only has Federal look under the hood of your car it will tell you on the sticker what it conforms too. I don't care what state you live in you may still have CA emissions on your car and that could make a difference on horsepower, gas mileage, and parts.