In general under the same conditions in regards to driving style i do get a better fuel economy in summer. However, during the warm season i'm much more often inclined to drive with the roof down, considerably faster than 210km/h and using the aircondition. That really drives up fuel consumption. Winter tires (mandatory here by law) are usually limited to only 210km/h which limits the speed, not to mention unfavourable conditions like snow, ice and so on limit speed even more.