Winter Gas Causing Drop in MPG?

[MadMax]

Here's the situation...since getting my 5K service back on 16 May, I've been getting between 36-38 MPG in my 08 auto hatch with Rike Raptor 215/45Z17 tires on it. But I got gas on 11 October and only got 34.6 MPG on that tank. I've been driving the Jeep a lot lately, so I didn't fill up again until two days ago; but this time I got even worse gas mileage...33.4 MPG.

Last year (I bought the car in Oct 08) I was only getting 32-34 MPG until I filled up in February, when it went up to 34-35 which it stayed at until I hit 5,000 miles and had the first service completed (with synthetic oil).

Now I know most of the gas around here now contain up to 10% ethanol, and that we might be getting "winter gas," which has lighter, low-boiling-point components are added to it to increase volatility, which may improve emissions and have a relatively high octane blending value but also is more expensive and lowers fuel economy. Allegedly, "the oxygen-bearing compounds displace fuel components, so it takes more fuel to get the job done. Normal gasoline has a stoichiometric ratio of 14.7:1, reformulated winter fuel runs between 14.3 and 14.4:1" (Source).

Now the kicker, it seems like my Jeep is getting better gas mileage! Lately I've been averaging well over 16 MPG, as compared to 13-15 during the summer. Some of that may be due to the fact that it is "buttoned up" (top and doors back on), which believe it or not makes it more aerodynamic versus with everything off. My driving habits don't seem to have changed, and since we don't really get winter weather down here in southcentral Texas (other than more rain), I can only attribute it to possibly being that the old I6 4.0L engine in my Jeep likes winter gas better than the I4 1.5L in my Yaris.

Does that make any sense? I am open to any other theories or observations...

Cheers! M2