What would cause a significant and sudden drop in MPG? I get around 22 in city driving.. but the last 2 tankfuls, I'm getting only 17.

I need expert opinions. Like Dave B is a consistent 1/4 machine, I am a MPG machine. I can tell 20 miles after a full tank what kind of MPG I am getting just by looking at the fuel needle. True to my suspicions, I was indeed getting terrible MPG during last 2 tankfuls.
The only thing that was changed on my car recently was the addition of RT cat and a catback exhaust.

I can't tell for sure if my car is sluggish than usual, which would indicate a probable knock sensor issue. I think it might be lil slower than usual but I'm not 100% sure. Due to DC metro's "tied for nation's worst" traffic, my butt dyno can't be tested during weekdays.

If my MPG doesnt improve with next full tank, then I really do have a problem.. The car was never like this in 130k miles..

HELP ~

 

not an expert opinion but check your fuel injectors. mine drops dead down when they become clogged.

 

Could also be your O2 sensors, spark plugs, Mods, mods and more Mods..

 

there could be a few factors. first, the temps are really dropping. what i've noticed is my mileage would drop noticeably when the temps are freezing. also, i'm almost positive that everyone's tire pressures are at least 2-3psi lower because of the cold temps. you should make sure they are properly inflated. you can add some concentrated fuel injection cleaner. that might help your mpg a little after a few tanks. lastly, the tank might not have been "topped off" at the pump. also, fuel quality varies from tank to tank and that will affect the overall performance of the engine. if it doesn't come back up after doing simple maintenance, you might be in need of a tuneup.

 

ohhhh. hmmmm.. It has been really cold lately.. I only drove in the mornings when I go to work and around 7pm when I come back home, lately. And its really cold in the morning and in the evenings.. That might explain the MPG drop.

As for the maintenance, my spark plugs are only 1000 miles old and the fuel filter is only about 10,000 miles old.

Bump.

What about knock sensor?? Would a bad knock sensor lower the MPG? Well I guess it would since timing will be retarded.

 

Oxygenated gas that is sold during the winter months will kill your mileage as well.

 

colder weather, denser air, proper ratio = more fuel. winter time gas, tire pressure, climate control use, etc etc etc. it all adds up.

 

for A while I've noticed my gas milage has been horible somewhere around 11mpg, I have no idea what it can be, should I reset the ECU?