Ever since last week my gas mileage went down 3 mpg and I'm suspecting it's because of the oxygenated gas supplied in the winter. Anyone else have any incite on this?

 

Its normal. Different formula of gas during the winter.

 

Oxygenated + Ethanol will result in at least 10% worse mpg than non-ethanol "straight" gas

 

Agree with the comments above... PLUS:
1.winter=cold air=more dense intake air=needs more fuel to burn (so really, your car might be making a tiny bit more power )
2.If you drive like me, you take pretty much any opportunity you get to do a powerslide/drift in the snow

 

this is good to know I thought my o2 sensor was going bad again

 

Ive notice the same mpg. The most ive ever gotten on a full tank was 275miles ? 95%city and 5%hwy

 

Really 3 mpg?! That's very high gas consumption and not normal if it is the reading from driving from full tank to 1/2 empty or full empty.

Mine is around 16 mpg, 50% hwy and 50% city, average temp 32 - 44 F degree (need heater on in the morning), using gasoline with 10% Ethanol. This is for my 20", staggered summer tires. Going to put winter tires on this weekend btw.

I would have the car checked if my car had that high gas consumption.

 

Well I was getting 20.8 - 21.8 overall now I'm getting 18.8 - 19.2 on the computer. Prob is just a mix of the gas and cold temps as we just had a 30 degree drop past 2 weeks and I do have a Stillen Gen 3 so air would be quite cold when it got to intake manifold. I have a hypertech installed too will try uninstalling that tomorrow to see if it makes a difference for next weeks commute.

 

I uninstalled the Hypertech on Sat and drove 45 miles so far and computer is reading 2mpg higher now so I think the Hypertech was causing lost mpg. I recently changed the Hyptertech tune to the Stillen one since I put a Gen3 in so I wonder if that was it.

 

Actually, I felt I should boast about this but I did get a 545 miles miles once and I actually chickened out because when I filled up I put in 17.9 gallons.