(sorry for the long post)
Lately I've been getting horrible gas mileage. The car is 00 Civic SI and I usually can get to about 300 miles before I need to fill up, however, lately I can only get to about 230. The only engine mod the car has is a homemade cold air intake. The total miles so far on the engine is about 50,000. The car does see alot of track time so normal wear tear does happen alot faster, however, when I do drive it on the street I rarely pass 4000 roms when going through gears. I'm guessing it may be that I need to change the spark plugs and possibly the fuel filter. I also tried to look for gas leaks but I couldn't find anything. Is there something else that I should be inspecting also, possibly fuel pump or rail?

 

Check your tire pressure. This is overlooked alot, but it could be a cheap fix.

 

I looked at that, but everything seems normal. I first thought because of the cold weather the gas mileage will be lower then usual, but this car has gone through 2 winters before. I'm going to try to do an oil change and a spark plug change to see if that'll make a difference. I did a search on this topic but everyone seems to have a variety of answers so I will see if my solution will work. In the meantime if anyone has any other ideas, please let me know.

 

New air filter, spark plugs, and a bottle of injector cleaner. You'd be amazed.

 

Thanks. I will try to see if that works.

 

This actually isn't a real fix just a suggestion, maybe you shouldn't shift so high try to bring it down to 2500 or 3000 RPMs maybe that will help with the gas mileage as well.