After you've dealt with the physical, fluid dynamics questions (aerodynamic drag), and the effects of clothing (flapping jacket), then you get down to the psychological questions.


How much does your face hurt when you're cycling into a headwind at freezing? How bad do your feet and hands feel? How hard is it to turn the cranks when you've got knee warmers wrapping around cold knees? Are your tires more stiff/less forgiving in cold weather?


It's probably not exponential, except perhaps converging to how you feel with every bit of winter gear you own on as you crank away in the coldest weather you've every cycled in.