I would choose 150s myself, since it's much easier to accelerate when
you're getting pitched around on the bumps. I have used 125s offroad
but it's a lot of work trying to avoid UPDs.

You may also want to consider 150s with a 24x3 tire. I prefer that
combo to anything with the 29" since it's much better at dealing with
the bumps. If you're only going 5-6 miles the increased rolling
resistance shouldn't be a big problem.


--
unibabyguy - Hunter-riding Municyclist
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