I recently completed my first tour, a 2,000 mile loaded touring event from San Antonio TX to New York State. Thirty four days, averaging 65 miles per riding day (not counting three rest days). The bike is an '89 Schwinn Voyageur, most everything replaced except the derailleurs and shifters. In the back I was running an 8 speed 11-34 tooth cassette. Up front a Nashbar 44/33/22 mountain bike crankset. Loaded touring, about 40-50 pounds of gear and water.
I never used the 44 tooth chainring at all, and the 22 tooth granny in combination with the 34 tooth sprocket was adequate all the way to New York. Once IN New York State however, some of the steeper grades really kicked my mass, I even ended up walking and pushing the bike an aggregate total of maybe one mile of that 2,000.
Besides walking however there were many long hills that would have been easier and much more pleasant to ride with lower gearing. Certainly this will be a necessity if I tour in the Mountain West.
My rear derailleur will not work with a larger than 34 tooth sprocket. As this is an older bike, I am not sure the chainstay geometry would work with a new derailleur/larger sprocket combo either. As it is the 8 speed cassette is used as a seven speed, as the chain rubs on the frame if I use the small 11 tooth sprocket (which in practice I never needed anyway).
Are smaller cranksets available? In a perfect world something like a 40/30/18 might work with the present derailleur.