Ha, Ha, ha... Oh wait, you're serious?
(Sorry, it's finals and I'm a little punchy.)
That depends on a few things: what gear you ride in the summer; whether you ride only on clear roads, or in the car snot and crunchies; what the wind is like; how much extra clothing you wear; how much bigger and lower pressured are your tires....
All that said, I rode my track bike with about 80" during the summer, and my winter beater with 68"-70" and frequently found myself working harder on the beater when the wind came up. Could've been the boots and 5 pounds of lights though.