Warm air is less dense than cold air.

Less dense == less oxygen

Less oxygen == less fuel needed for stoichiometric burn.

Winter/summer gasoline formulations make a difference too.

__________________
"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles