There's nothin' to it," he said. "Look, I'll show you. There's two rules you need to know to design these machines. First, the friction in every bearing is so-and-so much, and in every gear junction, so-and-so much. From that, you can figure out how much force you need to drive the thing. Second, when you have a gear ratio, say 2 to 1, and you are wondering whether you should make it 10 to 5 or 24 to 12 or 48 to 24, here's how to decide: You look in the Boston Gear Catalog, and select those gears that are in the middle of the list. The ones at the high end have so many teeth they're hard to make, if they could make gears with even finer teeth, they'd have made the list go even higher. The gears at the low end of the list have so few teeth they break easy. So the best design uses gears from the middle of the list.
From Surely you're joking Mr. Feynman, autobiography by Nobel Laureate and quantum physicist Richard Feynman
Also: Feynman on why train wheels don't need differentials: https://www.youtube.com/watch?v=WAwDvbIfkos