Are your daytime temperatures really down to the 20F in that document?  

50-60F temps, in my experience, produce the best fuel economy in most vehicles - the cooling system is not working hard, so the rad fan doesn't come on, you're not using the heater, so no parasitic lose there, the air is denser than at 77F or 90F, so the engine does less work to inhale air and gets more dense cylinder filling...

James
Ottawa, ON
'91 Westy Weekender 
(in the past:  '83 Rabbit L, 94 Golf turbodiesel (1.0L pre-TDI), 95 Golf GL 2.0)

> Date: Tue, 10 Nov 2015 13:31:50 -0800
> From: mjhart853@GMAIL.COM
> Subject: Re: Power & MPG Summer/Winter
> To: vanagon@GERRY.VANAGON.COM
> 
> Over the past ~3 weeks I've noticed a slight drop in power & mpg for my
> Westy (what power you may ask?). Specifically it feels like i'm fighting a
> headwind on the freeway at 50 - 60 mph.
> Searching for answers and hoping it is not a real engine problem (this 2WD
> Westy has 195k on the original engine), I followed a hunch on gasoline
> formulations & found the following;
> - the winter gasoline formulation for California has an average of 1.7%
> lower energy content (vs. summer) - due to added butane
> - the changeover date is Oct 31st (most stations switch over a little
> earlier)
> - other factors that affect fuel economy include air temperature (the
> daytime temperatures have dropped significantly over the past two weeks
> here)
> 
> http://www.afdc.energy.gov/pdfs/2876.pdf
> 
> Having read all this I am less worried that the engine is on it's last legs
> (I still have no really good excuse to spend large $$ on a new engine...)
> --
> Mike Hart
> Palo Alto, CA (SF Bay Area)
> 1987 2WD White Westfalia GL 'T3Westy'