I doubt you'll see a decrease in speed by going wider....if you're super concerned about it just make sure you buy GOOD wide tires and not just any wide tires.

A good tire will have a high TPI casing that is supple and provides a sweet smooth ride without sacrificing speed.

Check out the Grand Bois Cypress or the Panaracer Jack Brown


I am biased as I am a card carrying member of the Fat Tire lovers Society.