I rode from Nashville to Knoxville last Wed night with the lowest indicated temp on the bike being 30F. Find a way, if you can, to keep the wind off of your hands. I've found I can keep the rest of me sufficiently warm (not comfortably warm, mind you), but my hands and feet start going numb after an hour or so at interstate speeds. Hand dryers at restaurants are your friend and can warm your gloves and boots up nicely
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My personal threshold (no heated gear other than grips, mind you) is 50 and up is comfy. 40-50 is bearable for about as long as I need to ride. Below 40, and I can't take it for very long.