This is an interesting read although most of you already know what you need to do to increase MPG. The paper is directed at OTR trucks but most also applys to us.
Cliff Notes:
The most efficient drivers get about 30% better fuel economy than the least efficient drivers.
Ambient Temperatures
Air becomes more dense as temperatures drop, which increases air resistance. For every 10° F drop in temperature, aerodynamic drag increases by 2%. Thus, fuel efficiency will drop by 1%. (and I was thinking more dense air would give more air to the engine, didn't think about drag)
Speed
Above 55 mph, each 1 mph increase in vehicle speed decreases mileage by 0.1 mpg.
Fuel Blends
While blended fuels provide better startability and protection against fuel gelling than standard #2 diesel, fuel efficiency decreases. "Summer" fuel improves mileage up to 3% more than "winter" fuel.
FMB
(who drives above 55 most of the time)