Originally Posted by
Pugsly
Depends on the engine, but for me a lot. You are going to spin out a mtb earlier than the road bike, it's geared different. Also, fat slick tires are still a lot less efficient on the road than skinnny slick tires.
you can always change the gear range on MTB by going with a larger chainring.
since MTB can mean 26" or 29", 29" being the same thing as 700c, that and road has it's own smaller wheel variant of 650c, the gearing argument isn't valid.
for wheels, larger diameter = better rolling efficiency, so fatter slicks on 29'er/700c would give you even more rolling efficiency.