Modern vehicles come with a wide variety of options, any of which can be added to your package before you buy your new vehicle. One of the lesser known or lesser understood options available on trucks is the axle ratio. Choosing the correct axle ratio could be the key between your new vehicle being suitable for its intended purpose or not.
How to choose an axle ratio
Consider what the vehicle will be used for
In order to choose the correct axle ratio for you, first consider what you intend to use your vehicle for. Besides regular transport, will you be using it for anything else such as towing. If so, how much towing? Will this be a work vehicle with significant loads. Will there be long distance traveling without loads? Understanding axle ratio is important in ensuring your vehicle is equipped for its intended purpose.
What is an axle ratio?
The axle ratio is the gearing of the differential which links the driveshaft to the wheels. The ratio refers to the number of rotations of the drive shaft in relation to the number of wheel rotations.
An axle ratio is expressed with the driveshaft rotation followed by the wheel rotation. For example, a 3.21:1 refers to 3.21 rotations of the driveshaft to 1 rotation of the wheel.
Why is axle ratio important?
Axle ratio is important as it affects the capabilities and performance of the vehicle. As an example, changing the axle ratio of a vehicle will change its towing capacity without any change to the engine power. Just like using the low gear on a bicycle to go uphill, higher axle ratios improve towing capacity, pulling power and off-the line acceleration.
What are the cons of high axle ratios?
Despite the clear advantages of high axle ratios in towing and acceleration, there is of course a downside. A high axle ratio means more rotations of the engine/driveshaft for each wheel rotation meaning reduced fuel economy compared to lower axle ratios. When comparing a low axle ratio to a high axle ratio at a set vehicle speed, the engine will rotate more times per minute (rpm) at higher axle ratios and thus use more fuel. Vehicles like this might not be suitable for long distance journeys.
As a result, manufacturers offer multiple axle ratios where standard ratios will offer vehicles with better fuel economy and higher ratios for vehicles requiring additional towing capacity.
Often a towing package will be offered where an axle ratio upgrade is accompanied with additional upgrades such as improved vehicle engine cooling, trailer hitch points, upgraded heavy duty springs etc.
How do I choose which axle ratio is right for me?
To choose the correct axle ratio for you, you first need to decide what the vehicle will be used for. By default, it is better to go with the standard axle ratio as this will provide the best fuel economy. However, the standard axle ratio will have a maximum towing capacity. If the vehicle will be used to tow loads above the standard ratio capacity, or frequently at the maximum capacity, you should go for the upgraded higher axle ratio. The vehicle manufacturer is trying to build the most efficient vehicle they can so their recommendation on towing capacity is the best advice.
The axle ratio is not the only consideration. Modern vehicles offer additional transmission gears to reduce engine rpms while cruising on vehicles with higher axle ratios. Investigate this when doing your research.
Can you change the axle ratio?
So far we have only discussed factory optioned axle ratios. You can of course change the axle ratio in older differentials, however, you need to ensure that the new gear are compatible with the housing. There are a large variety to choose from and range from factory replacements to aftermarket upgrades.
Should you change the axle ratio for larger tires?
Another reason differential gear ratios are changed is when vehicles are modified to have larger (or smaller) tires than standard. Changing to a large diameter tire modifies the final drive ratio e.g. going from a 30” tire to a 35” tire will change the final drive ratio by 17%. In order to account for this change and restore the final drive to match the power band of the engine, you should reduce the gear ratio by 17%. A differential gear ratio of 3.07:1 should be changed to 3.55:1. However, if your goal is to improve off-road performance with large tires, you could change the gear ratio even lower, to 4.10. This will reduce your top speed and result in the engine running at a higher rpm at all speeds than before.
Most differential gear suppliers will be able to supply you with the advice you need to choose the correct gears for your differential. Contact Houston Rebuilt Axles if you have any questions.
What axle ratio should I choose for towing?
To maximise the towing capability of your vehicle, you should choose an axle ratio with the highest amount of rotations of the drive shaft per tire rotation. These lower gear sets will have increased low speed torque resulting in increased towing power. Again, changes of the axle ratio can drastically affect the drivability of hte vehicle. It will result in reduced vehicle acceleration from a dead stop whether towing or not, meaning a more sluggish vehicle while not towing. In addition, depending on the number of gears in the transmission, this can also mean a lower top speed and higher rpms at high speeds reducing fuel economy. Examples of axle ratios suitable for towing include 4.10:1 or 4.30:1
Summary
Axle ratio is an important aspect of purchasing or modifying a vehicle. It will dramatically alter how your vehicle drives and performs. Understanding why this is the case and knowing your vehicles’ intended purpose will help you make the right decision with your vehicle. If you need to calculate your gear axle ratio, use our free Gear Ratio Calculator. If you are not sure about axle ratios and which to choose, contact us at Houston Rebuilt Axles for advice.