| Ratio | Torque Multiplier |
|---|
What a Gear Ratio Is
A gear ratio compares how fast two meshed gears turn. Divide the teeth on the driven gear by the teeth on the drive gear and you get the ratio, the number of input rotations for one output rotation. It is the single most important number in any geared system, from a transmission to a differential, because it sets the trade between speed and torque.
Torque Up, Speed Down
A reduction ratio above one multiplies torque by that factor while cutting output speed by the same amount. That is how a low first gear or a numerically high axle ratio launches a heavy vehicle. An overdrive ratio below one does the reverse, spinning the output faster than the input to lower cruising RPM and save fuel at the expense of torque.
From Teeth or From RPM
You can find the same ratio two ways: count the teeth, or divide input RPM by output RPM. They always agree, which is handy when you can measure shaft speeds but cannot see inside the case. Chaining ratios, such as a transmission gear times an axle ratio, multiplies them into a single overall final drive.
Frequently Asked Questions
Which gear is the driven one?
The driven gear is the output, the one receiving power. The drive gear is the input. Driven teeth divided by drive teeth gives the ratio.
What is a good axle ratio?
It depends on use. Numerically higher ratios like 4.10 favor acceleration and towing; lower ratios like 3.08 favor fuel economy and top speed.
How do multiple gears combine?
Multiply them. A 3.20 first gear times a 3.73 axle gives an 11.94 overall final drive in that gear.