So I just had my head gaskets fixed because I was told by multiple shops that they were leaking. Well everything generally seems to be running pretty nicely since the repair I am having a couple issues since this repair. I have the base model 2.5 subaru Legacy from 2009 

1. The fuel mileage has dropped from around 26-27mpg on average to 22-23mpg on average

2. There seems to be a different smell coming from the exhaust.

 

These repairs were finished 2 weeks ago so I don't know if there is a break in period for some of this stuff or not. Just looking for some opinions as to whether I have something else going on here.

A few things could be going on here, the battery would have been disconnected, and as such the computer "forgot" all it's data on trends and gas mileages. And if you live somewhere cold, you're now probably on a winter blend of gas, letting your car warm up more, etc. And as such you're going to get worse gas mileage. Before the headgaskets were done, your average was based on your city and highway driving, etc.

If you've only been doing lower gas mileage driving lately, yea, I can see that reflecting in the average gas mileage more since it doesn't have as much data to average over.

No unfortunately not, I've actually driven primarily on the highway since that time as I drive 30 our of 35 miles that way to get to work. Also I know it was not disconnected because it still had the mileage from my "B" trip odometer on it.

Hate to even bring up the possibilities, but it could be a small intake leak, or, more hated, a notch out on the timing belt. Just thinking more of what can happen when that work is done. Yes, a plug could have hit the floor and closed the gap up etc, but it usually a bit closer to the work done. Good luck, hope you find your gremlin. Jim

Lets start with a basic, where do you live, how cold has it been.

 

After that look for a vacume leak or the timing belt off a tooth (common even to the pros).

 

How is the idle?