l have a 2005 subaru wrx, with only a custom cat back exhaust on it.l have noticed that im getting 15 miles per gallon, when im supposed to get like 26.any possibilities why im getting such bad mileage.It was never this bad, l dont drive crazy.

stay out of boost..

Well your WRX stock mpg was 20/27 which in 2008 epa standards would be 18 mpg in the city and maybe 24 mpg on the highway. Depending on how you drive(Red light drag races, constantly in boost rpms, AWD launches..) you could possibly be getting 15 mpg in the city but only you know the way your REALLY driving.

No ideas asides from the ones tyrone said. If you are worried about fuel economy though get new spark plugs - sometimes that helps.

Air filter. Low tire pressure. Skewed oxygen sensor or maf.

Check engine light on? Lead shoes? Neighbor with a siphon?

Your driving habits must be bad or they dont suit the car. All Subys are thirsty cars anyway...Get a stealer to check the electonics to see if there is a fault. check boost & fuel pressure

Nobody gets 26mpg out of a WRX. The quoted figures are impossible to acheive in real life driving.

15-20 in the city is about right, depending on driving style, A/C use etc. Fuel octane rating also effects consumption so make sure you are using the good stuff when quoting figures.

l have a 2008 Forester turbo and it gets around 22 but with a lot of open road driving.

You do not by a WRX for the economy...

If you have to start your car a lot that sucks a lot of gas too. But l always get 26mpg on the highway no matter how fast l drive.