Gas milage

i have a 2004 impala ls with 100,000 miles and i only get about 240 mile out an entire tank. thats less that only about with 15mpg. i just change the spark plugs, new K&N airfilter, just change oil, and anitfreeze. i drive about 50% city and 50% highway. i dont know if its that Catalytic Converter. if so where can i get it checked out. Any help is very valuable.

Not enough information to go on here, engine size? How do you calculate your mileage? There are just too many things that contribute to poor mileage to diagnose easily. Dealer is most reliable for checking cat defects, if not defective the charge is hrly/time for that dealer. Request cost before you authorize the work. Should be covered under warranty if defective, but I am not sure of the "04" model year. Others here may know for sure.

How you're measuring mileage is also important -- if your tire diameter deviates from the factory size even just a little, your odometer will either read short (larger dia.) or long (smaller dia.), making your gas mileage calculations off as well.

Low tire pressure and carrying extra weight will impact gas mileage negatively full time, so check the simple things first before looking at powertrain problems.

It's also a bad idea to include "city" driving as any indication of gas mileage because it is always likely to be lower than you expect -- when stopped and idling, you get Zero MPG, but gas is still being used, so stop signs, traffic lights and traffic significantly impact mileage.

The best way to measure mileage is to fill up, and immediately get on the highway and drive about 50-70 miles (more is better) of highway only (a round trip is preferrable) ... get off and fill up again (preferrably at the same pump) -- note the miles traveled and gallons required to fill up -- divide miles by gallons to get miles/gallon.

Loading the engine with accessories like the alternator (engine cooling fans, headlights, radio, HVAC blower, rear defrost), and A/C has more of a negative impact in city driving than on the highway.

I am having the same problem on my 05' with 77K with the 3.4L. I've had the computer checked but my mechanic says everything is good. Since it has been really hot the engine is weak and the transmission does not want to shift. Could be clogged cat expanding like others mentioned. I'll let you know if I replace it and if it helps.