I'll go from 43 highway summer to 35 winter ..
Just this am it was 29*
Boy does it run good though ...its the ice on the seat I don't like ....

 

Gas has been reformulated for years between summer and winter. Winter fuel has a lower flash point (usually more butane) so the engine can start easier. Due to this, there are less BTU's in a gallon of winter blend then summer blend, hence lower mileage.

 

The fuel mileage may be less, but the bikes will tend to run much stronger. Will feel like a bigger engine with the cooler air.

 

mine really goes down - 6 to 7 mpg - plus here in California we have the worse gas in the world anyway - but i still love this state for better or worse - here it is 11/26/12 7 & it's T shirt weather - tom