Gear ratios explain how a vehicle trades engine speed for wheel torque. In a car, gears let a fast-spinning engine move a heavy vehicle from rest, climb hills, and cruise efficiently. A low gear gives strong pulling force at the wheels but limits speed.
A high gear allows higher road speed with less engine rpm once the car is already moving.
A gear ratio compares how many times the input gear turns for one turn of the output gear. If a small driving gear turns a larger driven gear, the output turns more slowly but with more torque. If a larger driving gear turns a smaller driven gear, the output turns faster but with less torque.
Transmissions and final drives use these ratios together to match engine power to driving conditions.
Key Facts
- Gear ratio = teeth on driven gear / teeth on driving gear
- Output speed = input speed / gear ratio
- Output torque = input torque x gear ratio, ignoring friction losses
- A 4:1 gear ratio means the input turns 4 times for each 1 output turn
- Low gears have high ratios, giving more torque and less speed
- High gears have low ratios, giving less torque multiplication and more vehicle speed
Vocabulary
- Gear ratio
- A comparison of input rotation to output rotation in a gear set.
- Torque
- A twisting force that can cause rotation, often measured in newton-meters.
- Input gear
- The gear that receives power first from the engine or another rotating shaft.
- Output gear
- The gear that sends changed speed and torque to the next part of the drivetrain.
- Final drive
- The gear reduction between the transmission and the wheels, usually located in the differential or axle assembly.
Common Mistakes to Avoid
- Thinking a higher gear ratio means higher vehicle speed, which is wrong because a higher ratio usually reduces output speed while multiplying torque.
- Forgetting the final drive ratio, which is wrong because wheel torque and wheel speed depend on both the transmission gear and the axle ratio.
- Assuming torque multiplication creates extra energy, which is wrong because gears trade speed for torque and real systems also lose some energy to friction.
- Mixing up driving gear and driven gear teeth, which is wrong because reversing them gives the inverse ratio and changes the predicted speed and torque.
Practice Questions
- 1 A 12-tooth driving gear turns a 36-tooth driven gear at 3000 rpm. What is the gear ratio and output rpm?
- 2 An engine produces 180 N·m of torque through a 3.5:1 first gear and a 4.0:1 final drive. Ignoring losses, what torque reaches the axle?
- 3 Explain why a car starts in first gear instead of top gear, even though top gear can produce a higher road speed.