How a car's odometer counts · OakCrest Academy

🚗 How a car's odometer counts

In a mechanical odometer, a cable turned by the gearbox, in step with the wheels, spins a worm, a screw-shaped gear that turns the ones wheel slowly.
Each kilometer advances the ones wheel by one digit: 1, 2, 3, and so on up to 9.
When the ones wheel rolls from 9 to 0, its two carry teeth engage a small gear called a pinion.
The pinion advances the tens wheel by exactly one digit: ten ones make one ten.
Every wheel does the same for the next, so 0999 becomes 1000 with all four wheels turning at once: three roll from 9 to 0 and the thousands wheel moves from 0 to 1.

New words

odometer: the instrument that counts the total distance a vehicle has traveled
place value: the value a digit has because of its position: ones, tens, hundreds, thousands
carry: when a place reaches ten, it returns to 0 and adds one to the next place
pinion: a small gear that passes motion from one wheel to the next
worm gear: a screw that turns a toothed wheel, greatly reducing its speed

🏠 Try it for real

In the car, note the odometer reading before and after a short journey. Which digit changed, and did any digit to its left change as well? Explain why.

💭 Talk about it

Why does the tens wheel move only when the ones wheel passes from 9 to 0, and at no other time?

Now explain it back: point at each part and say what it does.