temperature & fuel mileage

[foxxinabox at wideopenwest.com (Foxx (in a box))]

   i've been griping about the MPG in my '85 and can't seem to find the 
problem. with the cold weather i was usually lucky to get 24 MPG. i 
think there is a vacuum leak from the brake booster (hear a hiss when i 
press the pedal) and tried pumping it vigorously (the pedal). i was in 
for a pleasant surprise tonight when i filled up and got 29.4 MPG. does 
the warm weather really help the running of the car that much or has the 
rubber simply expanded somewhere and stopped a leak? just seeing if 
anyone has past experience. thanx.