temp-mpg relationship

[hah at srv.net]

>At the risk of starting a Thread From Hell, would any of our more
>tech-oriented listers like to take a whack at why, as temperatures rise in
>the late winter into the early spring (in the Northeast U.S., in this 
case),
>gas mileage improves?  I was averaging 20.1 mpg in February.  I was
>averaging 21 mpg in March.  Now I'm nipping at the 22 mpg mark.  Type of
>driving and type of fuel are not factors: I have not altered my driving
>routine -- about 75 percent highway and about 25 percent city.  I have not
>altered the kinds or octane of the fuel I use.

Are you sure that your friendly local refinery has not altered the vapor 
pressure or composition of its gasoline output? "Winter gas" can be a 
quite different mixture than "summer gas", same advertised octane, same 
"brand"...

What is the average temperature for the three months? Cold starts when it's 
colder have to bring the engine to operating temperature through a bigger 
delta-t, probably takes longer to reach closed-loop operation = more fuel 
used in open-loop when it's colder. Especially if the engine has an air-oil 
cooler (only cools oil when over thermostat temp) rather than an oil-water 
heat exchanger (also warms oil from coolant from cold start).

Just idle speculations, HTH
Henry Harper
http://www.srv.net/~hah
1991 200 quattro, 112k (air-oil)
1988 GTI 16v, 217k (oil-water)