OK thanks for the input guys.

Now I have a few questions, though.

All of these comments "seem" related to the added rotational weight, higher inertial resistance etc. big tires = more work. You're hosed on that regardless.
But isn't that irrelevant (independent) of the gearing? All the gearing does is change where in that rotational speed you put the RPMs, and have to shift... right? I can see how once you are moving, the gearing sets where you are in the power band at a certain speed. E.g. you don't want to find yourself at only 1.5k when doing 35 mph... no power.
But if the relative relationships change correctly, so that you have, say, 2500 RPM at that same speed of 35mph, then why would it matter what gear you are in?

And why hard on clutches? Please explain.