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This time of year my mileage really takes a dive... I've attributed it to the following things:

1. Increased air resistance with colder temperatures. The air (being ever so slightly more dense) is ever so slightly more difficult to travel through. Although more dense air enters the combustion chambers, the car still has to travel through it, requiring matching fuel to burn that dense air and even more fuel to propel the body through it
2. Colder tires - increased rolling resistance compared to summer
3. The engine requiring more thermal energy to run the same operating temps as the summer (up to 60F cooler)
4. Any aerodynamic modifications... the muffler strikes me as a key component for under-tray airflow. If you've made this mod, the colder temps will exasperate your fuel economy as exiting air will cause a pickup-truck-like venturi under the rear bumper
5. Increased warm up times, requiring different fuel maps for longer periods
6. Potholes jarring your alignment (particularly toe) to out of spec. Out of spec toe readings will mess it up.

I don't think it's out of line for a 15% increase in fuel consumption given all of these factors (whichever apply to you)... 15% off of 37mpg, leaving you around 31.5mpg.

Paying for the crushed walnut treatment for the intake valves is one option, (you would have noticeable power loss from now with gummed up intakes... have you lost a lot of power??) however I'd probably look to nature/aerodynamics/drag to be the cause...
 

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you do realize I'm talking about getting 5-6MPG less than I got in the same/similiar conditions last year vs this year? I think a lot of people are missing that point...
My normal winter drop for this car was 2 MPG.
How many times has that been said now?!
If your main suspect is the intake, rip it apart (or have the stealer do it) and tell us what you find. Head to an alignment shop, check your parking brake, check for sticking calipers, check your plastic underbody tray. Hell, it could be some play in the front bushings that allow the alignment to change under load, and thus at highway speeds causing more drag. If you've done all this, then there must be no reason.

If this was a car that had been around for more than 1 year, there would probably be an answer to your concern. Perhaps you can be on the leading edge.

Obviously nobody has anything left to say to you as we'd all be at a loss to offer any further advice. You will be the architect in solving this problem...
 

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I went from 33mpg (12/2013) to 26mpg (12/2014), and it's warmer weather this year than last. In fact, in order to achieve the 26mpg, I had to drive 8mph slower than I normally do.

In the meantime, I've observed the vehicle follows ruts in the road like there's no tomorrow.

It also feels like I'm riding on 4 unicycles for tires... almost like the wheels are loose (they are not). I have to assume the bushings are shot, and the wheels are experiencing some deflection at speed/load. The tires have started to wear unevenly as well. No pull to either side, just odd behavior over all surfaces.

Check your plastic undertray for anomalies as well... Most definitely these things are warranty issues. I would provide you an update, but I just unloaded the car on something else.
 

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Besh, I checked the aero panels first as ticker recommended early in the thread, all good under the car. How many miles did your 6 have? I went through a period of REALLY REALLY weird steering feel at about 25K and it sorted out by 30K and the car feels good again. Also I'm due for a set of tires at 36K (buying next week in fact), and that certainly contributes some to those factors.
The vehicle had 22,000 of the hardest miles you could ever put a vehicle through. Here in Canada, frost heaving destroys roads, cold snaps muddy the engines, sand and salt corrode and cause cancer. I had driven through snow deep enough for the front lip to form a snowplow... on a regular basis it seems. They dump enough sand and salt on the roads to make it a gravel texture. And everyone still crashes (for some reason).

The paint on the front of the vehicle looked like a shuttle going through re-entry. The paint on the rocker panels had worn down to bare metal. The various potholes (sometimes the road is the pothole) had deformed the tires slightly (and likely the 19" wheels). No wonder the bushings were shot. The body was flexing like a young Arnold Schwarzenegger and creaking like an old mattress at a brothel.

A good road here is one that doesn't smash your head off the side window on a regular basis. I exaggerate, but you get the idea.

That being said, I will say that this is the most fragile vehicle I've owned in a long time. When I first had it, I wouldn't have guessed it. Solid as an anvil, and a great ride to boot. The deterioration was quite shocking. The 2003 Nissan Maxima I had before this seemed to be twice as hardy, despite looking like it was made of tin (front control arm bushings and sway bar bushings through 3 hard years... nothing more).

I had a huge list of unresolved problems (axles, brake rotors, wind noise, engine startup noise, bushings, strut noises, clunks and bangs, trunk latch busted, throttle issues, power loss, horrible fuel mileage, seat heaters not working - the list goes on), partially due to the dealership... "car feels fine, nothing wrong" and due to the fact I live a ways away and had a hard time continually hassling them (better things to do).

An all time high of about 39mpg with ease to struggling to get 26 and no immediate recourse?

It was a relief to unload this one, sorry forum!!
 

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Right, don't argue about $3.28.

If you keep your car for 200,000 miles, and average 30mpg (I don't believe your BS claims of 41mpg at 85mph or whatever garbage you said earlier) over the life of the vehicle (which you might be lucky to accomplish as it ages).

200,000 miles / 30miles per every gallon = 6666.66 gallons (the devil just smirked)

And let's say premium is $0.20 to $0.50 more per gallon... You're spending $1,333.33 to $3,333.33 over the life of the car. If not more, because premium is likely more than $0.50 per gallon extra.
 
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