High gears are called high, And have high rpms, Because of the numeric gear ratio. All for example, First, being a high gear, is say, a 3:1 ratio. Meaning the drive gear spins 3 times(high rpms, more mechanical advantage) for one rotation of the driven. At 5th, The ratio is say 1:1, a low gear. The drive and driven spin the same. Which is less rpms and less mechanical advantage as opposed to 3:1. And overdrive means that the ratio is less than one. .75:1 for example. And supra2nr, For your explanation, A smaller guy running faster and a stronger guy running slower pulling the same weight, Are doing the same ammount of work, But one faster and one with more force. That is the important part.

And also, My n/a was at about 3500 rpms going 70 lol. It sucked balls on the highway.