Two things:

1) On tours and sometimes on day rides, I find myself on a variety of
surfaces, including coarse gravel in campgrounds, plus some stretches
on the road, fine gravel on rail trails, rough pavement, pavement
with small amounts of dirt on it, etc. Sometimes I ride on grass at
campgrounds as well.

2) Seems to me a tire with some tread might be able put more distance
between the tube and the road than a slick tire. Or if the slick
tire has the same tread thickness, it would be heavier than the
treaded tire.

So for my purposes, I would look for a tire with a smooth center,
some tread beside it, and relatively flexible sidewalls, maybe a
Schwalbe Marathon.

Just before a recent tour, I switched from some treaded Haro BMX tires,
which have never gone flat in urban riding, to some Comp Pools. I didn't
notice a huge jump in speed, but I don't ride very consistently, speed
wise. I _did_ notice a tendency for the tires to pick up glass and wire
bits, such that on a week long tour, I did flat from a piece of wire and
removed a deeply imbeded piece of glass that would have eventually
worked its way to the tube.



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