55MPG during the summer??? I think ur lying.

 

This link seems to explain it well to me...

Winter blend gasoline rolling out, drops in MPG possible - Gasbuddy Gas Prices

 

I track mpg every tankfull on my smartphone, it's interesting where I get 38 or so one tankful, then drop to 34 the next, then back up to 38, then back down as the winter fuel switch takes place and I fill up at various stations.

 

I've never seen an analysis to actually explain the drop in a way that convinced me with data. Here's the standard reasons:

Winter gas contains less energy per gallon. I don't know why, actually. I think it's to reduce offgassing emissions at the temperatures expected in the season. I think California has "fall" and "spring" gas but I could be completely wrong there.
Winter air is thicker. MAAYaybe. I'm not sure I believe it makes a difference.
Your car is less aerodynamic in the winter.
You spend more time idling the car waiting for it to warm up. Or, you leave it on longer when stopped for short periods of time, to get the heat.
You use the A/C more - mostly because of the moisture and the need for the defroster.
There's more friction from cold lubricants in the winter.

Feel free to debate which have the most effect. I've no idea. I'd go with gas change, idling times, AC use and lubricants, in that order. In fact, just to guess, let me put up some estimates; since I don't have to source this at all :P
Winter gas: -5%
Warmup/idling time (not moving the car): -5%
AC use: -3%
Cold lubricants (overlaps with warmup time): -1%

Multiply those out and you get -13%, or (just as an example) a drop from 38 to 33 average MPG which, from my numbers, is right about on the money.

 

I have a 2011 Sport manual trans that I drive on long highway commutes. It has always gotten the best mileage (say 40 with really easy/careful driving) in the summer but THIS winter has been really bad for mileage.

Last winter my mileage dropped to around 36-37 but the last couple of tanks I've only gotten 31-32. My car has 55K miles on it and I have done no service other than oil changes and I'm wondering if there is a problem that needs addressing.

 

Holy shit, you guys get 40 in the summer?! I thought the laws of thermodynamics were opposite. During the winter time here in Texas I get better mpg but shit I've never gotten 40 mpg in either cases. You must by driving like grannies or have an ETC to help out.

 

Gasoline has a lower energy density in the winter due to the more volatile cuts put into the mix. Summer-blend gasoline has a higher vapor pressure as a result which "theoretically" reduce mileage.

 

Colder weather lowers mileage in several ways.

Higher tire rolling resistance. (nearly double!)
Higher aerodynamic drag. (10% higher)
Cold engine = more fuel burned to warm it up.
Colder denser air = more power! but it takes more fuel to do that
If you don't watch it, your tire pressure goes down, which drags more and costs fuel.

Just those first two account for about a 20% loss in mileage.

Summer can reduce mileage with AC, but even then it's better than winter, and spring and fall are just perfect.