Final Drive Ratio
What Is Final Drive Ratio?
Final Drive Ratio refers to the last set of gears that connect a vehicle’s engine to the driving axle. The ratio is the number used to represent how many times the transmission needs to rotate the drive shaft in order to rotate the tires once.
Final drive ratios affect the acceleration, towing capacity and fuel economy of a vehicle. Lower ratios provide better fuel economy at highway speeds, but higher ratios are necessary for grater acceleration, hauling and towing. Manual changes to to gears can influence a vehicles speed of acceleration and top end speed.
KEY TAKEAWAYS FINAL DRIVE RATIO
- Affects acceleration, towing capacity and fuel economy of a vehicle
- To calculate the Final Drive Ratio, divide the number of teeth on the ring gear by the number of teeth on the pinion gear
- Lower ratios result in higher top stop
- Higher ratios result in better acceleration
Understanding Final Drive Ratio
There are two gears mounted on the driving axle of a vehicle: (1) the pinion gear, and (2) the ring gear. Both the pinion gear and the ring gear affect the performance of a vehicle. These gears reduce the speed of the transmission’s gears at the axle and divide that power (torque) between the two driving wheels.
The pinion gear is the small gear at the end of the drive shaft connecting to the larger ring gear, which rotates the drive axle. The Final Drive Ratio is also called the “Axle Ratio”.
1) reduce the speed at the vehicle’s wheels;
2) divide the torque between the two wheels.
To achieve maximum fuel economy you want an axle ratio lower in the number of drive shaft rotations (pinion gear) for every tire rotation (ring gear). This level of axle ratios result in lower rpms, which mean better fuel economy for your vehicle. Lower axle ratios are not suitable for towing and hauling heavy loads. See Above.
An axle ratio that is higher in the number of drive shaft rotations for every tire rotation is more suitable for towing. The lower gear sets provide more low speed torque, or towing power, at the rear wheels. Therefore, less acceleration is required to move your load from a dead stop. These gear sets are better suited for towing and hauling heavy loads.
Low Final Drive Ratio
Generally, a lower final drive ratio will lead to less torque at the wheels but a higher top speed. This means:
- Less work for the engine
- Greater fuel economy
- Lower towing capability
High Final Drive Ratios
A higher final drive ratio will result in more torque at the wheels but a lower top speed. Since torque is what helps the vehicle accelerate, a higher final drive ratio will offer better acceleration. The reason a higher ratio will help accelerate a car faster is because the engine can spin up faster and creating more leverage to turn the wheels.
Higher final drive ratio will mean:
- More work for the engine
- Less fuel economy
- Faster acceleration rate
- Greater pulling power
Calculating the Final Drive Ratio
The straightforward way to calculate the Final Drive Ratio is to divide the number of teeth on the ring gear by the number of teeth on the pinion gear.
For example, if the ring gear has 30 teeth and the pinion has 10, take the number 30 and divide by 10 to get the ratio of 3:1. This means the pinion must rotate three times to make the ring gear rotate once, which in turn rotates the tire once.
If you are looking for an online tool to calculate your Final Drive ratio, you can find one here.
Related RV Terms
For related RV terminology, please reference the RV definitions below:
GVWR: The safe weight maximum a vehicle can operate with, including payload.
Payload Capacity: The maximum amount of cargo weight that can be added to a vehicle.
Travel Trailer: A non-motorized compact mobile home that can be towed behind a vehicle.
Trailer Hitch: The connector piece used to tow a trailer behind a vehicle.