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.

http://www.everytime.cummins.com/every/pdf/MPG_Secrets_Whitepaper.pdf (from
RORT)

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)