First, I'd compare friction...with an old bike (or a mistreated newer one) its easily possible something on yours is sticking that isn't on his.

Then, I'd count gear-teeth on both bikes, and input the tire sizes and gear ratios on to a calculator (I think sheldonbrown.com has one or a link to one)
Its possible that along with the tire size there's some gear ratio differences that keep your bike from being ridden as fast.

Posture?