[As Paul commented, I began swapping tires around like crazy very soon into the trip. I'm now a veritable expert at this.]
I began the trip with what my LBS thought should be
fine...something called Invert IIs. (Clearly more for roads than for
fire roads, occasional goat trails, tracks through the meadow and the
like.) Since "Donkey Xote" (my mt. bike) lost his virginity re:
off-road on the first day of the trip (woe is me, I wanted to get
some off-road experience before I left...but that accident...the best
laid plans and all of that...), I had nothing to compare it to. But I
skidded and slid and pedaled along thinking that everything was fine,
and wondering, "why do people think this is so much fun?" 
(Picture credit: StrayDawg.) At the end
of day 2, I swapped my spare tire onto the back. (My spare was a
Ritchey Speed-Max [folding] which I had picked up at a bike shop just
before leaving...no rhyme or reason for getting that particular tire
other than I knew the name (Ritchey) and the price was right, $24 or
so. I had difficulty determing why I should pay nearly as much for a
bike tire as I had just paid per tire for my Volvo wagon.)
At the end of day 3, when we rolled into Whitefish (and to wonderful bike shop there...Glacier Cyclery), I stopped in "just to chat a bit about tires." (Translate: find out what the hell I should be doing about tires.) The mt. bike gurus there suggested leaving the Speed-max on the rear and getting a Z-max (also Ritchey) for the front. Fine. I did that. Things were only slightly better. My steering was still all over the trail. (In retrospect, the advice wasn't good...it may have been better to put the speed-max on the front and the z-max on the back right then and there. But they were racing mt. bike gurus and probably were of that mindset in their recommendation to me.)
(I didn't even know that tires can have directional rotation - Paul pointed that out to me AFTER I had mounted the Speed-max the first time - much less have dedicated front/rear tires.)
I struggled along with that for the next week. In Helena, I was getting ready to do more tire tweaking with the local bike shop (Eric at Great Divide Cyclery) when one of our group offered to sell me his spare tire (he was leaving because he could only be gone for two weeks) - a Cyclepro kevlar foldable ("neutral" tread). All the trip experts and friends (Paul, Bob/Sally, Al, Tom, Alan) had something to say about where to put which tire. The end result was the obviously rear tire (z-max) should be moved from front to rear, the speed-max would become the spare, and the "neutral" kinda-knobby would become the front tire. Well, that improved things markedly!! (And have I learned more than I ever wanted to know about mt. bike tires! )
Next year: Conti Navigator Pro has been recommended. If not those, at least something with differing front & rear treads, and more gnarly than what I started the trip with. :-) Sure does slow you down on pavement, however. But I'll take the slower pavement (I was so slow anyway, that another mph or two couldn't possibly make enough of a different to worry about) for more fun on the trails.]
Copyright Judith J. Colwell, 1997. All rights reserved.
Last Updated: May 14, 1998