10+ MPG Loss

[linux2]

So for some reason my 2010 Chevy Cobalt XFE with the 5 speed manual tranmission is now only getting about 28-30MPG no matter what I do. I was getting 38 city and 42 highway. I'm trying to figure out why.

The only modifications the vehicle has to it is 1 different tire (right front tire is different, but didn't decrease the MPG when initially put on about 8 thousand miles ago).

I have replaced the air filter with a K & N air filter. I'm wondering if that could be causing the problem. MPG loss started around putting this filter in, but it also got a lot colder which could have something to do with it.

Finally, my exhaust smells like eggs, but it has since I bought the car and the dealership says "we can't reproduce the odor". Although you could ask anyone who has been near my car if they smell something and they will always say "kinda smells like eggs". No error codes and the car seems to run fine. Just probably the cat is broken and has been since I bought it so it is hard to say if this is finally becoming a problem.

Anyways, I was thinking it could be the air filter, but I'm looking for any other input. It has gotten a lot colder out and I lost a lot of MPG since it started getting colder, but it doesn't seem to add up. It was in the 50's a couple of days ago and my MPG was still terrible. Even if I coast down all hills and am super light on the pedal I can only get about 31 MPG. and about 29 if I drive the car hard. Doesn't make sense to me.

Thanks in advance to any input!

+++++++++++++++++++++++
The car has about 19K on it. Oil changed regularly and tire pressure is right where it should be. Alignment is fine. Maybe I should check the plugs?

 

[reyesa21]

i dont know whats wrong but i dont know what im supposed to be getting but im getn 21 city and like 29-31 hwy. i feel like i shud b gettn bettter too lol anything to gain mpg?

 

[cobalt123]

spark plugs, fuel filter, proper tire air pressure

 

[RedDead]

you can also put some awesome vinyl on your car that gives you at least 15 hp!!!!!!!!!!!!!!!!

 

[linux2]

i dont know whats wrong but i dont know what im supposed to be getting but im getn 21 city and like 29-31 hwy. i feel like i shud b gettn bettter too lol anything to gain mpg?

Sounds very similar to my problem. Let me check my plugs later and I'll let you know what that looks like. I'll also check the fuel filter and let you know. Seems a little early to be changing that, but who knows.

 

[sLAsh]

Additions to list
Stuck thermostat
Winter gas

 

[reyesa21]

Sounds very similar to my problem. Let me check my plugs later and I'll let you know what that looks like. I'll also check the fuel filter and let you know. Seems a little early to be changing that, but who knows.

alright cool, and plus mines an 09 sedan if that makes a diff.

 

[USMCFieldMP]

The below are all the things that I'd say are potentially at fault.

-Cold Air

-spark plugs
-proper tire air pressure

-Winter gas

 

[Tuner923]

Sounds very similar to my problem. Let me check my plugs later and I'll let you know what that looks like. I'll also check the fuel filter and let you know. Seems a little early to be changing that, but who knows.

changing fuel filters doesn't necessarily go by mileage. it mostly pertains to the quality of fuel you are using. and no i don't mean octane. a lot of gas stations use fuel they buy from suppliers with the lowest bid meaning less quality (dirty and watery). some companies have even been known to water down the fuel to maximize their profits.

 

[reyesa21]

changing fuel filters doesn't necessarily go by mileage. it mostly pertains to the quality of fuel you are using. and no i don't mean octane. a lot of gas stations use fuel they buy from suppliers with the lowest bid meaning less quality (dirty and watery). some companies have even been known to water down the fuel to maximize their profits.

so what is good fuel or where at? if you know