Car differential

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

In Transmission by Josh S0 CommentsLast Updated: May 20th, 2019

car rear axleThe differentials and axles are essential components in transferring power from your engine to the rear or front wheels.

Internal combustion engines operate through the combustion of air/fuel mixture. When the pistons move they in turn power the driveshaft.

How the driveshaft works

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 driveshaft by the presence of universal joints on both ends. On one side of the driveshaft is the transmission slip yoke and the other end has axle connection. The rotations are fashioned in a counter-clockwise manner.

The rear axle has the braking system and rear suspension. Power is received on the rear axle from the connection to the driveshaft in the middle. This is then delivered to the wheels through a gear system. This is held through a 90-degree angle. The pinion gear will deliver power from the driveshaft to the hypoid gear. Gear oil is used to ensure lubrication. Due to the heavy load that the hypoid gear system is subjected to the lubrication uses a splash system instead of the typical pump system to deliver oil.

In this instance, the pinion bearings are partially submerged in oil. The reason being that they rotate faster than the other bearings and should there be insufficient lubrication the whole system will suffer.

Piston Gear vs Ring Gear

The piston gear system is smaller than the ring gear and therefore rotates at a faster rate. This gives birth to the axle ration. Typical axle ratios can be numbered as 3:42:1, which means the piston gear system has to rotate 3:42 times for every one rotation 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 selecting the right axle for your truck you need to understand the amount of load you will be towing and the speed you want on the given RPMs. If it is larger then you need higher torque delivered by higher axle gear ratios.

Lubrication of the rear axle

Car differentialThe connection between the piston and ring gears requires a lot of lubrication otherwise the two will grind against each other causing friction. The piston ring rotates at a faster rate than the hypoid ring. Whenever the car is not moving the piston gears are partially submerged in oil. In addition, the part of the differential and pinion bearings are also submerged.

This enables the oil to be supplied to where it is needed the most – in the pinion bearings that are rotating at a faster rate than the rest of the moving parts. If you are suffering from low lubrication the first part to suffer will be the pinion bearings. Many truck owners wonder why the tapered roller bearings are not used for pumping the oil. The reason is that oil is moving from an area of a smaller diameter to a larger one. In this instance when it arrives at the pinion bearings they pump it back to the sump.

The sizing of the return port ensures that the oil does not remain dormant between the pinion seal and the tail bearing. In some older truck models, you will have such kind of display and this ensures that oil is not adequately pumped into all areas of the differential. You also find that the oil is contaminated more. The reason being that the ring gear takes the oil from the sump and this contaminated oil is pushed to the cavity between the piston bearings.

Whenever you are doing maintenance for you truck it is important to always clean out this area. Oil is also needed to lubricate the axle shaft bearings and seals. Oil is moved by the ring gear and differential case and distributed to the pinion bearings and inside the axle center regions. The ring gear is often located to the left side of the axle tube meaning it is not centered. This means this side receives more oil than the right side. In this instance when you are traveling at 35mph the left side will receive oil but the right side will only receive this oil when you hit speeds of over 50mph.

Axle Housing

The axle housing should not be pressurized or exposed to a vacuum and this means the housing needs venting. Most of the axle housing will use a set of jiggle style caps for venting. This have the added benefit of having very little metal. As you move the car temperatures inside the axle begin to rise. If this pressure is not contained it will pressurize the seals. Pressurized seals are not good as they allow the oil to move past the seals. The seals are also more prone to wear and tear and you may find yourself replacing them more frequently. To avoid all these, the axle will have a bypass that gets rid of some of the pressurized air.

The axle housing at all times carries a lot of weight. Besides holding the vehicle weight the axle also holds the suspensions. This means that the axle has to be created strong enough otherwise the car weight crashes it. In addition, to all this, there is the braking force that is generated by the wheel cylinders and calipers. The motion, if not contained, can damage the axle.

Lastly, the axle has to resist the force from the rotation of the prop shaft that rotates in anti-clockwise fashion. In this scenario the force is unbalanced the right wheel receives an upward pull while the left side receives a downward pull.


The piston gears rotate at a faster rate than the hypoid gears. This rotation means that you need to find the right axle rotation for your car. If you want a truck that can carry a heavier load then you will need to a piston gear that delivers higher torque. On the downside, this kind of gear ratio consumes more fuel.

If you do not carry a lot of loads and are interested in faster acceleration then go with a lower gear ration.

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