I've had my 03 CLS for about a year and a half and haven't had any problems. I recently got an oil change and noticed right away my gas mileage went down significantly. I usually get about 300-350 miles per tank, and am now getting 250 and below. Could this be due to the oil change, or could something else be wrong here?

 

mines gotten pretty shitty lately as well. I need to get a tune up though. Im not sure what you're problem might be.....

 

just a thought, did they use the same viscosity as before? if not, that might be your problem. i can't even see 300-350 miles per tank. after i added my sound system, i lost about 25 miles per tank. sucks

 

My car runs much better and efficiently in cool-cold weather. Doesn't like 90 degree weather.

 

And that's added to the fact that most gas has 10% ethanol now (at least up here)...

It could be alot of things, but I wouldn't think that oil would make for 100 miles a tank difference...

 

thing is, I took it do the dealer a couple months ago to get its scheduled maintence, had them change the oil w/ 5w20, which I really don't think they used cause I noticed my mileage started to go down about 20-50 miles per tank. Now I'm getting even less after my last oil change. I've always put 93 gas in it and put a can of seafoam in the tank about 4 months ago...I'm just not sure what else it could be...It has been pretty hot though, mid-high 90s...maybe o2 sensor?