text: can anyone explain to me which each gear for each situation on a mountain bike?   there's no one gear for any situation.   whether road or mountain you just need to get out there and get to know your bike.   do you switch to a high gear on downhills? mmmmm maybe. if your legs are tired you might want to "spin it out" on the way down.   do you gear down going uphill? mmmm maybe. there's what i've been told is the "magic gear" you're climbing, legs giving out or sore, stand and click up 2 gears higher and climb, had enough? click down two and hey, two lower gears i didn't know i had.   even the exact same route sometimes you feel fresh, climb in a higher gear, sometimes not so hot, lower than before.   there's no one gear ... just log off the dang computer and get outside and ride and get to know your bike.