I own a 2009 droptop (6 speed manual, base engine) in LA. I am not an aggressive driver and used to get 33-34mph mixed driving and 38-39 mpg highway driving (driving 70-80 mph). A few months ago in December, I noticed a sudden drop in mpg (now getting 29-30 mixed & 34 mpg in hwy driving) almost overnight. I tried several things (trying higher octane, getting wheels balanced, confirming proper tire pressure) but nothing worked. Dealer ran diagnostic but couldnt find anything wrong either. Has anyone had similar experience? What was the fix? I calculate mpg as 'miles driven since last fillupn'/'gallons it takes to fill up', so it should be accurate

 

Did you change tires around then? I lost a few when I went from 175 rubber to 205/55/15.

 

New tires also lose MPG generally when compared to the same tires that have been worn-in.

Did your commute change at the time? Did the weather get a lot colder at that point? Was anything else changed?

 

does it need a tune up? alot of time spark plugs and dirty filters will kill your MPG

 

did you change gas companies? my car only likes chevron/texaco. i get 30-31 in my mcs, but when i had used shell i would get 28-29. and i filled up at a few times so it wasnt just a fluke or anything.

 

Vacuum leak possibly if the dealer didn't check for that. I had a Subaru Impreza that had a bad vacuum leak before the air flow sensor and it only got 10MPG. After everything was fixed it got around 30MPG. Also, a dirty air flow sensor could be the case if there is no vacuum leak. You could try cleaning it with some maf cleaner you get from Autozone.

 

Running rich? Maybe an O2 sensor?

 

Would definitely take a look at changing the spark plugs if you haven't done that lately.

Replaced air filter? How is your coolant doing - no leaks, proper level - so it keeps the motor running at the proper temp?

 

Thank you for all your input. There was no change in tires or to my driving habits. It happened in December so I thought maybe the mpg would improve when the weather got warmer but it didnt. I am due for an engine oil change so when I stop by I will ask the dealer to check for spark plugs, air filter and for the vacuum leak (I am still under warranty). Hopefully one of those will fix the problem.

 

I had something similar. Ended up being a fault break caliper that was dragging. Not enough to notice but enough to drop my MPG by about 15%. Covered under warranty :-)

 

Hope you get it figured out keep us posted.

 

Dealer ran computer diagnostic bit didn't find any error messages so refused to perform a visual inspection (like the brake caliper or the spark plugs like other owners suggested). However, by chance, they changed the compressor (in accordance with maintenance schedule) and that immediately brought the mpg up to 32-33 mpg range in usual mixed driving, which I am satisfied with. It is still a tad lower than what I was getting but I will take it!

 

Right before my timing chain noise I noticed ready good MPG, then timing rattle at start-up brought MPG down. When timing components were replaced MPG went back up.