Originally Posted by Sloth
A bumpy one, Helmet head. Smaller tires get lower rolling resistance by increasing pressure (larger radius tires have to deform less to roll and thus do have inherently lower RR, see Bicycling Science.)
The higher pressure is not only uncomfortable, but the tire does not deform for bumps and thus energy is lost acelerating the bike - and you - over the bump.
A lower pressure tire just absorbs the bump.
Well, yeah, but I manage to avoid all the steets with cobblestones on my commute...
Seriously, what kind of bumpy roads are we talking about?