For some reason, my MPG has dropped from roughly 21-22 to 18-19. Worthy changes are mainly a new set of tires. The temperature has also dropped slightly to about 40. Furthermore, my car was recently stuck in the mud with no visible damage to the undercarriage, but if there's anything that might be affected I thought it'd be worth mentioning. Any ideas?

 

1. Winter gas
2. Are you basing your mpg-numbers off the number of miles traveled divided by the number of gallons put in, or by using the MID?

 

I've heard tires can make a difference if:

1. they are heavier than the old ones

2. they are wider than the old ones (ie: went from 235 to 245)

 

Mine seems worse also. Seems I'm hitting the gas station more. I have been driving more with family visiting but still. Is the winter gas that much of a diffence?

 

Right... do NOT use the numbers the MID is reporting to you. Calculate your mileage yourself: fill your tank until the gas pump stops. I can squeeze an additional 0.25 gal or so before it's full. Reset a trip odometer. Drive the car until your near empty, then refill to the approx. same level (i.e. until it stops, then add the same amount you did, 0.25 in my case). Now divide the miles as per the trip odometer by the gallons.

Here are a few factors that will affect mileage, there are probably more:

-Errors due to incorrect tire size (i.e. now your odometer is counting too fast)
-Outside temp (colder being worse)
-Winter/summer gas formulations (winter being worse)
-Weather (rain, snow, etc.)
-Driving style (fast starts, high speeds, etc.)
-Cruising speed (you'll get better mileage at 65 than you will at 80)
-Traffic density (frequent slow down/acceleration)
-Head/tail winds
-Tire rolling resistance (performance tires are worse in general since they are made to be 'sticky' and provide you with traction. A/S tires can also hurt mileage, but hey, you bought them for their saftey and grip didn't you?
-Inflation (under inflated = worse)

 

Cold weather = Bad gas milage