I picked up my HJC carbon fiber helmet on Friday and I was a bit dubious as to the size. I have been riding with an HJC XL helmet and it is comfortable. My new helmet is large and it fits snugly against my cheeks.
To my surprise, after measuring my head and comparing the charts, I should be wearing a large (not XL).
A greater surprise is that my vision, at high speeds, is now very clear since there is no longer any shaking of the helmet.
So check the charts and be sure to wear the helmet at the shop for at least 15-20 minutes to see if it gives you a headache.
Fitting the right lid to your head makes a difference. Size does matter!!