Here’s a diagram of the Model T transmission.
On this diagram, gears B, D, and E are planet gears. They freewheel on pin b but they are a single unit and must spin together as they are a single unit. These are called the “triple gears” and there are three triple gears on three pins on the flywheel spaced 120 degrees apart.
Gears A, F, and G are three independent sun gears. Gear A is the output gear and it is driven by gear B. Gear F is the low speed sun gear, and brake band J locks it stationary when the transmission is in low.
Gear G is the reverse sun gear and it is locked stationary by brake band H when the transmission is in reverse.
Brake band K brakes the brake drum to stop the car and inside the brake drum is a multi disk clutch which locks the output shaft to the flywheel shaft for direct drive when the transmission is in high.
The output gear A has 27 teeth and so does the planet gear B that drives it. a 1:1 ratio.
The low sun gear F has 21 teeth and it mates to a triple gear D with 33 teeth.
The reverse sun gear G has 30 teeth and it mates with the triple gear E with 24 teeth.
If the engine is not running and you turned the low speed sun gear F one complete revolution, it would result in the output gear A to turn 21/33 or .6363 turns in the same direction that the sun gear was turned. When the sun gear is locked stationary and the planet gear F walks around it for one revolution, in effect the planet gear’s orbit tries to turn the output gear one revolution but the rotation of the planet gear from gear D walking around stationary gear F causes gear B to rotate it backwards 21/33 of a revolution. One revolution – 21/33 of a revolution leaves you 12/33 of a revolution being delivered to the output gear or a gear reduction ratio of 2.75:1
In reverse, brake band H locks gear G with 30 teeth stationary. It mates with gear E which has 24 teeth for a ratio of 30/24 or 1.25:1.
When it is locked, rotation of gear B tries to turn the output gear backwards 1.25 turns for each orbit of the gear resulting in the output gear being driven backwards 1/4 turn for each engine revolution. The reverse gear has a 4:1 reduction ratio, which is why a lot of people used reverse to climb really steep hills in these cars.
One more thing you will notice is that all the gear teeth are integer multiples of 3. This allows all three triple gears spaced 120 degrees apart to be identical and interchangeable and still line up with the teeth on the other gears.
Image by Peter H from Pixabay