During the winter with all around town stop and go driving I get 10.5 to 11.5. During the summer about 14-16. Highway usually mid 20's.

 

Cold weather has the opposite effect on mileage. When it's -20*C out it takes forever for the engine, and especially the transmission, to heat up. That means the oil pumps are having to work much harder to push viscous oil/trans fluid through the passages. The result is you need more throttle and burn more fuel to maintain a given speed.

Now that we've warmed up a bit around here, my mileage has increased from 12.9 L/100km to 10.9 L/100 km (or increased from 18.x mpg to 21.x mpg) with a mix of about 50/50 city/highway driving.

The thing about colder, denser air is that the throttle does not have to be open as far to pass a given mass of air into the engine. The same amount of fuel will be burned for the same mass of airflow regardless of temperature or density (we'll just ignore cold start enrichment as that is active for only a short part of any trip). Once the drivetrain has warmed up, air temperature really has no effect on fuel economy.

You're correct about the effect of cold air on power. I took a look through some logs from last summer and this January. Wow, what a difference. In July, with an IAT of 30*C, cylinder charge was just .62g/cyl @WOT. On January 11 with an IAT of -38*C, the cylinder charge was .75 g/cyl, an increase of about 21%. That's like running three pounds of boost compared to the summer! (I'm at about 740m, or 2,300 ft, altitude, so I get about 10% less cylinder charge than at equivalent sea level temperatures.)

The decrease in air density with increased temperature is why I won't run a cone filter in the engine compartment. I've ducted cold air from the fender well into the stock air box instead, and just use a drop-in K&N.

 

There is a cruise power button that illuminates amber. Make sure that's on. Then get up to speed (greater than 30mph) and press the set button. you should see a green light illuminate on the gauge cluster indicating you have set CC. Once you're in CC mode, you can use the + and - button to speed up and slow down w/o using the pedal...and the circled-X button to cancel the cruise, especially helpful when you know there is a cop ahead but don't want to be jamming on your brakes to tell them you were speeding.

 

Well Im on the highway all the time. I live in South Chicago and work in North Aurora and usually end up getting 250-275 before I need to fill up (but I play around a lot and my LS1 is bone stock lol).. but I also don't let the fuel gauge get under 1/8 of a tank. Plus I make that trip to Ohio every year to go visit my dad and you should only have to fill up the tank fully twice to get there... but on those trips I usually fill up every when Im down to 1/4 of tank just to be safe. Plus ur coming from Springfield so Id just plan like $120 each way just to be very very very very safe.

 

i was banking on about 250 for the whole trip. thanks alot guys and also i may switch over to amoco (BP) for my car.

 

MCSS mileage is worse than the 4 doors.

I get no more than 23 on the highway @ 74 in cruise. I don't race this car, so I switched 6 months to mid-grade. Haven't noticed the .10 loss in acceleration following the minivan that's doing 20 under the speed limit and dragging it's breaks for a half block before it gets to the next red light or twist in the road.

Needless to say, my car's lifetime average is 18.7 now. 50 / 50 driving on my 12 mile commute to work.