The gear ratio is 5:1 (five to one), and that means that the pinion must make five revolutions to turn the spur gear one revolution.

You’ll use 5th and 6th gear in a car for: Driving at high speeds on main roads, dual carriageways and motorways. Many years ago, most cars had 4 forward gears. 5th gear was introduced as a fuel efficient cruising gear. Some cars also have a manual 6 forward speed gearbox.

Gear 5 allows Luffy to fight with complete freedom, which means that he should be able to use aspects of his other Gears all at once. This would explain why Luffy is so drained when Gear 5 deactivates. Unfortunately it also means that Gear 5 likely shortens Luffy's life span just like Gear 2.