Sudden decrease in fuel mileage on my 03 civic. went from like 330-340 a tank to 260ish. Any ideas? Any info is appreciated

It could be something such as a faulty oxygen sensor. That is one thing that could cause a large loss in gas mileage. It could me many different things, definitely get it checked out.

Check out these two sites:
http://answers.yahoo.com/question/index?qi...02095847AAS8tGb
http://www.edmunds.com/reviews/list/top10/...64/article.html

These might be of help to know the reasons and tips on how to improve.

Also when was the last time you changed the fuel filter? That could also cause this problem.

I think it's probably significant that the fuel decrease was sudden.

It could be slightly affected by tire pressure and alignment, but I'd bet it has something to do with the air/fuel mixture, as rs634158 suggests.

If you notice a gradual or even sudden decrease in your car’s gas mileage (assuming that nothing drastic has changed in your driving habits) there are several things that could be going on with your car that might be causing the drop in fuel economy.

Maybe you’re driving around with a dirty or clogged air filter, fouled spark plugs or under inflated tires. There’s also the possibility that the culprit could be one of the last things you might suspect - your car’s brakes.

It shouldn’t come as much of a surprise, but if your brake pads are constantly engaged it’s going to take much more energy for your car to get up to and maintain traveling speed. This extra energy will need to come from burning/wasting extra fuel.

There are several things that could cause your brakes to constantly drag:

* One or more of your brakes’ calipers may not be fully retracting after you have disengaged your brakes.
* The emergency brake doesn’t fully release due to a malfunctioning cable.
* One or more of your brakes’ retaining springs or clips may be defective and causing the brake pad to not release from the rotor.

Obviously these are just a couple of the potential problems. However, because your car’s braking system is absolutely critical to your safety and well being, please take your car to a professional to have the problem properly diagnosed and taken care of.

While you have your car in the shop, it might not be a bad idea to have the mechanic look over the rest of your car to see if there’s anything else that they might be able to repair that might help improve your car’s fuel economy.

Even though it might be a pain to fork over the money now, if you keep your car properly maintained over time, you should be able to recoup much of the cost by saving money at the pump. Hope this helps.. Goodluck.

I would start by asking if your sure someone is not getting in your tank at night.... Then your brakes, then your injection system !

Now how many miles, you driving habits, modifications ?