How to Choose the Right Axle Ratio for your Truck/Pickup

Car Axle Ratio

Differentials and axles are essential components in the transmission of power from your engine to the rear or front wheels.

Internal combustion engines work by burning an air-fuel mixture. When the pistons move, they in turn drive the drive shaft.

How the driveshaft works

Car Rear Axle

You need the power to be delivered from the engine to the rear axles. The driveshaft is the link between the two. You identify the drive shaft by the presence of universal joints at both ends. On one side of the drive shaft is the transmission fork, while the other end has an axle connection. The rotations are counterclockwise.

The rear axle has the brake system and the rear suspension. The power on the rear axle is absorbed by the connection to the drive shaft in the middle. It is then transmitted to the wheels via a transmission system. This is held at a 90-degree angle. The pinion delivers the power from the driveshaft to the hypoid wheel. Transmission oil is used for lubrication. Due to the high load, the hypoid gear system is subjected to, the lubrication uses a spray system instead of the typical pump system for oil delivery.

In this case, the pinion bearings are partially immersed in oil. The reason for this is that they rotate faster than the other bearings, and if the lubrication is insufficient, the entire system suffers.

Piston Gear vs Ring Gear

The piston gear system is smaller than the ring gear and therefore rotates faster. This results in the axle ratio. Typical final drive ratios can be numbered as 3:42:1, which means that the piston gear system must rotate 3:42 times for each revolution of the ring system.

  • You need higher ratios to deliver a higher torque = Lower speed @ RPM.
  • Lower numerical ratios mean you have less torque but better fuel efficiency as the engine does not overwork.

When choosing the right axle for your truck, you need to know the amount of load you will pull and the desired speed at the given RPM. If it is higher, you will need a higher torque delivered by higher axle ratios.

Lubrication of the rear axle

Car Differential

The connection between the piston and the ring gears requires a lot of lubrication, otherwise the two will rub against each other and cause friction. The piston ring rotates at a faster speed than the hypoid ring. Whenever the vehicle is not moving, the piston gears are partially immersed in oil. In addition, the part of the differential and the pinion bearings are also submerged.

This allows the oil to be directed to where it is needed most – the pinion bearings, which rotate faster than the other moving parts. If you suffer from low lubrication, the first part to suffer will be the pinion bearings. Many truck owners wonder why taper roller bearings are not used to pump the oil. The reason is that the oil moves from an area with a smaller diameter to an area with a larger diameter. In this case, when it reaches the pinion bearings, it is pumped back into the sump.

The sizing of the return connection ensures that the oil does not remain at rest between the pinion seal and the end bearing. Some older truck models have such an indicator, which ensures that enough oil is pumped in all areas of the differential. You will notice that the oil is more contaminated. The reason for this is that the gear rim picks up the oil from the sump and this contaminated oil is forced into the cavity between the piston bearings.

Whenever you carry out maintenance work on your truck, it is important to always clean this area. Oil is also needed to lubricate the axle shaft bearings and seals. The oil is moved through the gear ring and differential housing and distributed to the pinion bearings and within the axle center areas. The ring gear is often located on the left side of the axle tube, i.e. it is not centered. This means that this side receives more oil than the right side. In this case, if you drive at 35 mph, the left side receives oil, but the right side only receives this oil at speeds above 50 mph.

Axle Housing

The axle housing must not be placed under pressure or vacuum, which means that the housing must be vented. The largest part of the axle housing is fitted with a set of loose caps for venting. This has the additional advantage that very little metal is present. When you move the vehicle, the temperatures inside the axle begin to rise. If this pressure is not contained, the seals will be pressurized. Pressurized seals are not good because they allow the oil to move past the seals. The seals are also more prone to wear and you may need to replace them more often. To avoid all this, the axis has a bypass that allows some of the air under pressure to escape.

The axle housing carries a heavy weight at all times. In addition to the vehicle weight, the axle also holds the suspensions. This means that the axle must be strong enough to prevent the vehicle’s weight from causing it to fall. To all this is added the braking force generated by the wheel cylinders and brake calipers. The movement, if not contained, can damage the axle.

Finally, the axle must resist the force generated by the rotation of the drive shaft, which turns counterclockwise. In this scenario, the force is unbalanced, the right wheel gets a pull-up, while the left side gets a pull-down.


The piston wheels rotate faster than the hypoid wheels. This rotation means that you have to find the right axle rotation for your vehicle. If you want a truck that can carry a heavier load, you need a piston transmission that delivers more torque. On the other hand, this type of transmission ratio consumes more fuel.

If you don’t carry many loads and are interested in faster acceleration, choose a lower gear ratio.

Written by: Magnus Sellén

Founder, owner & main author of Mechanic Base. I have been repairing cars for more than 10 years, specialized in advanced diagnostics & troubleshooting. I have also been a drifting driver and mechanic for over 7 years.