I can climb at about the same rate on the bike on 6%-7% grades as on foot on a 15% mountain trail, with a pack about the same weight as the bike, which is about 20 lbs. That's for climbs of over 1000, for which I have good records. However I think I can keep it up longer on the bike. Appropriate hiking trails for fast altitude gain are about 15%. Good climbing roads around here are 6%-7%. I don't have appropriate gearing for multi-thousand foot 15% climbs on the bike, besides there aren't any around here.
My favorite cross training for cycling is hiking and backpacking once a week in summer. Helps.
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