Decrease in gas mileage =[

Track it for a few fill-ups and see if it's a trend or if it's just one off tank. And yeah, actually write the mileage and gallons down and do real math.

My MPGs dropped off pretty starkly with the onset of cold weather and I suspect the changeover to winter gas blends a couple of months ago. It's gone from my usual 18 MPG to a consistent 15.

Stop pushing the loud pedal so hard.

With the temperature dropping over the last month, i can think of at least 3 reasons.

1. Colder environment means unless you have added air, your tire pressure will be lower, resulting in lost fuel economy through increased rolling resistance

2. Colder environment means air is denser which raises the total aerodynamic drag (not to be confused with your drag co-efficient). Your car has to do more work to push the air out of its path

3. Colder environment, especially in ice and snow, means your tires never reach normal operating temps which further increases their rolling resistances.

4. So many other things that could have happened that I can't think of.

If you are seriously concerned with your fuel mileage, the best way to improve it is to remove as much restriction in both the intake and the exhaust path. Get it tuned and specifically tell the tuner you want an economy tune added. Add tires with lower rolling resistance ratings. On top of that, if you have aftermarket gears over 3.55, adding a 6 gear with a .50 ratio will do worlds for your mpg. Even with everything I have done to my car including the supercharger, I will still get over 30 mpg on the highway. At 200 miles, I am barely under 3/4 of a tank which is still stock size. if you have questions, let me know.

Another side effect of cold, dense air is that pumping losses are increased at cruising since it takes less throttle opening to allow the same mass of air into the engine.

Between the temps in the teens and 20's, long periods of idling when checking for leaks after my intake swap and lots of bursts of WOT acceleration enjoying the results of my intake swap, I'll be lucky to crack 200 miles on this tank