On my Cross Check, running 29X1.95 Bontrager XR-1 tyres, with 170mm
crank arms I used a 42 X 22 setup. I spin a fairly fast RPM so I
could easily ride a steady 17mph pace without thinking much. This
allows me to climb hills rather nicely and still cruise along the
flats (I don't remember many of these). But I always bring a couple
of gearing options with me so I can change my mind based on weather
the night before. In drier conditions than last year, I may have
opted for a 42 X 21.
Keep in mind that I am not riding to race anyone besides myself. And
I have yet to finish a TI (3rd attempt this year) And I wouldn't have
a prayer keeping up with true racers for long. I plan on running the
same 42 X 22 gearing this year with a slightly narrower 29X1.8 tyre.
I know that deep into the ride, my knees will thank me for keeping the
strain off them. Again, a slow cadence for me is 80 RPM so that makes
a huge difference in gear ratios.
Good Luck!!