Originally Posted by
prathmann
That would be my guess as well. Just because both have tires of the same size (622mm diameter, 28mm width) doesn't mean they are of the same quality or performance. A tire with an inefficient tread design and stiff sidewalls can have far more rolling resistance than one with smooth tread design and supple carcass.
My guess would be tire weight or wheel weight since it makes a big difference in how it feels to spin it up even if it's as little as 100g. A quick look shows the Nashbar tires are half the TPI of the tires on the Giant.