Paul, 40mm Schwalbe Marathons. The GAP is extremely well maintained, we could have easily done it on 28mm road tires. Saw some singles on 23mm racing tires.
However, we did the GAP along with the C&O Canal trail, which is very rough in places, and often poorly maintained. The 40 mm Schwalbes we’re good for that, and I wouldn’t do it with narrower tires if I had the option.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.