What is a gear ratio?

A big gear turning a small gear makes it spin faster. A 40-tooth gear has 5 times as many teeth as an 8-tooth gear. Turn the big one once and the little one spins 5 times! Swap them round and it is 5 times slower. That number, 5 to 1, is the gear ratio.

Try Propeller Push Car for free

Complete MightyCog booklet collection: 82 PDF booklets for $12.99. Instructions only; individual booklets remain free.

Explore the printable pack

Try it yourself

Put a mark on both gears. Turn the big gear once and count how often the small one goes round.

Build it: 32 builds that teach gear ratio

  • 🌀 Propeller Push Car: Push the car and the propellers on its back spin three times faster than the wheels! (level 1 of 5, about 12 minutes, 8 steps)
  • 🔢 Odometer Cart: Five turns of the wheels make the flag go round just once — a cart that counts! (level 1 of 5, about 15 minutes, 9 steps)
  • 🌉 Speed-Cancelling Gear Bridge: A 40–8–40 gear train: the middle turns five times, but the ends stay together. (level 2 of 5, about 15 minutes, 5 steps)
  • Gear Speed Lab🧪 Gear Speed Lab: Two cranks, two gear pairs: one makes things FAST, one makes things STRONG. (level 2 of 5, about 20 minutes, 13 steps)
  • 🔬 Compound Counting Bench: Two five-to-one stages multiply: count 25 input turns for one output turn. (level 2 of 5, about 20 minutes, 7 steps)
  • 🎡 Crank Cruiser: Turn the handle and the car rolls — every turn of your hand spins the wheels three times! (level 2 of 5, about 18 minutes, 9 steps)
  • Sliding-Gear 2-Speed Cart🕹️ Sliding-Gear 2-Speed Cart: Your first real gearbox: slide the crank axle to switch between strong gear and fast gear! (level 2 of 5, about 22 minutes, 8 steps)
  • 🔥 Big-Wheel Dragster: Giant back wheels, tiny front wheels — the same spring pushes this car much further! (level 2 of 5, about 22 minutes, 10 steps)
  • 💨 Gear-Up Speeder: The spring engine sits up high and turns a BIG gear, which spins the wheels faster! (level 2 of 5, about 25 minutes, 11 steps)
  • 🐢 Pull-back Tortoise: Slow but super-strong: a spring car that can climb up a book! (level 2 of 5, about 30 minutes, 12 steps)
  • 🌪️ Ripcord Top Launcher: Pull a toothed rack and watch two gear stages turn the top. (level 2 of 5, about 20 minutes, 18 steps)
  • 🌀 Flywheel Friction Car: Vroom it on the floor, let go — and it keeps on rolling! (level 2 of 5, about 25 minutes, 5 steps)
  • 🚧 Parking Barrier Gate: A long striped barrier that lifts with a tiny turn — thanks to a heavy counterweight. (level 2 of 5, about 20 minutes, 9 steps)
  • 🥚 Egg Beater: Turn the handle and two whisks whizz round in opposite directions. (level 2 of 5, about 20 minutes, 12 steps)
  • 🔋 Motor Buggy: Your first car that drives by itself — with a forwards/backwards switch and headlights! (level 3 of 5, about 30 minutes, 20 steps)
  • 📡 Spinning Radar: A motor-driven radar dish that sweeps round and round on a turntable — with a blinking light. (level 3 of 5, about 25 minutes, 15 steps)
  • 🎠 Chair-Swing Carousel: Crank faster and faster — the chairs fly out wider and wider! (level 2 of 5, about 35 minutes, 16 steps)
  • 🎚️ Two-Speed Pull-back: Choose your gear before the race: CLIMB or SPRINT! Slide the back axle to swap gears. (level 3 of 5, about 35 minutes, 13 steps)
  • ⚙️ Clutch-Gear 2-Speed Car: Push the gear lever and a driving ring grabs a different gear: 1st climbs, 2nd races! (level 3 of 5, about 45 minutes, 18 steps)
  • 🎛️ One Motor, Knob Steering: The motor drives, YOU steer — turn the knob as you walk beside it. (level 3 of 5, about 40 minutes, 21 steps)
  • 🛠️ Ratchet Car Jack: Crank the handle — click, click, click — and lift a toy car. Let go and it stays up! (level 3 of 5, about 35 minutes, 15 steps)
  • 🔥 Twin-Piston Engine: See inside a car engine: two pistons race up and down in turns! (level 3 of 5, about 40 minutes, 22 steps)
  • ⏯️ Geneva Step Counter: Turn the handle smoothly — the pointer moves in jumps: click… wait… click! (level 3 of 5, about 30 minutes, 9 steps)
  • 🥁 Cam Drummer: Crank the handle and two drumsticks play a beat all by themselves. (level 3 of 5, about 30 minutes, 18 steps)
  • 🌀 Spirograph Drawing Machine: A machine that draws flowers! The paper spins while an arm swings the pen. (level 3 of 5, about 45 minutes, 19 steps)
  • ↩️ Reverse-Gear Truck: The motor only spins one way — the gear lever decides forwards or backwards! (level 4 of 5, about 55 minutes, 18 steps)
  • 🏁 Three-Speed Car: Two little gearboxes in a row make a big one: crawl, cruise or fly! (level 4 of 5, about 60 minutes, 20 steps)
  • 🧗 Worm-Drive Crawler: Super slow, super strong: worm gears drive both axles and it never rolls back. (level 4 of 5, about 50 minutes, 20 steps)
  • 🌍 Sun-Earth-Moon Orrery: Push the arm round: the Earth circles the Sun, spinning as it goes, with the Moon in tow. (level 4 of 5, about 35 minutes, 12 steps)
  • 🚧 Twin-Motor Dozer: Wide caterpillar tracks, one motor per track, and a blade that cranks up with a worm gear. (level 5 of 5, about 100 minutes, 33 steps)
  • 🧭 South-Pointing Chariot: Push it anywhere — the pointer keeps pointing the same way. No magnets, just gears! (level 5 of 5, about 75 minutes, 34 steps)
  • 🕰️ Spring Escapement Clock: A ticking pin-wheel clock powered by the pull-back motor's spring. Tick… tock… tick… (level 4 of 5, about 50 minutes, 22 steps)

In the classroom

Common engineering questions

How can I try gear ratio at home?

Put a mark on both gears. Turn the big gear once and count how often the small one goes round.

Which builds teach gear ratio?

32 MightyCog builds teach it: Propeller Push Car, Odometer Cart, Speed-Cancelling Gear Bridge, Gear Speed Lab and Compound Counting Bench and more. Start with Propeller Push Car: 8 steps, about 12 minutes.

Ideas that go with it