Changing a Rear Axle Gear Ratio
Choosing the Right Truck Differential Ratio for Towing, Fuel Economy
Aug 10, 2009
When towing a trailer and the truck seems lack power, especially uphill, lowering the axle gear ratio may help. If the truck’s burning too much fuel, increasing the axle gear ratio could increase fuel mileage per gallon. Naturally there are some negative affects when changing a trucks axle ratio.
If the axle gear ratio is lowered, the truck will burn more fuel. If the axle gear ratio is increased, the truck will loose power, which may be especially noticeable uphill.
Basics of the Trucks Powertrain and Gear Ratios
The trucks gear ratio begins with the engine and ends at the wheels. Changing the trucks axle gear ratio is about making the wheels spin faster or slower as it pertains to the revolutions per minute (RPM) of the engine.
A slower turning engine equates to better fuel economy. A faster turning engine will create more horsepower for towing and climbing hills. There are generally two other factors that affect wheel speed between the wheels and the engine, the trucks transmission and axles(s).
Engine RPM
- The trucks engines RPM is based on how may times the engines crankshaft turns in a given minute. At a consistent highway speed say at 65 mph, the engines RPM can differ from on truck to the next depending on many factors like transmission type, axle ratio and tire size.
Transmissions Affect on Engine Speed
- In lower transmission gears, the engine creates more RPM and more horsepower, which is important for accelerating when taking off. In straight drive, the output of the automatic transmission is turning at the exact same speed as the engine. In a manual transmission the same rule applies in the gear before overdrive.
- In overdrive, the output of the transmission is actually turning at a faster speed than the engine. Overdrive is great for fuel mileage, but generally not good for towing a heavy trailer, especially uphill.
Axle Gear Ratio Affects on Engine Speed and Wheel Speed
- Changing the trucks gear axle ratio, affects the engine RPM at a given speed and the wheel speed. As an example if the objective were to save fuel, the axle ratio would be changed to a higher gear ratio. If the trucks gear ratio is 4.10 and was changed to a higher ratio like 3.55, the engine would turn approximately 15% slower, everything else being equal.
Replacing the Axles Ring and Pinion to Change Axle Gear Ratios
Of course one way to get a different axle gear ratio is to replace the rear axle, but that could get pricey. The most common method is to change the axles ring and pinion. The ring and pinion is what creates the ratio in the axles differential.
In the illustrated photo, the ring and pinion on the left is for a 3.55 ratio axle, the one on the right is for a 3.08 ratio axle. The two pinion gear heads are a different size and the 3.55 pinion has 11 gear splines whereas the 3.08 has 13 splines. The ring will also have a different amount of splines and must be matched with the pinion.
The important thing is that the ring and pinion must be compatible with the axle. The truck dealer, axle specialist or off road truck shops should be able to tell what gear ratio ring and pinions are available. When looking for a replacement ring and pinion, bring all the numbers off the metal tag on the axle for easier identification.
4 X 4 Axle Gear Ratios
When changing axle gears ratios in a 4 X 4, the front and rear axle ratios must be the same ratio when operating in four-wheel drive. If the front and rear axles are different ratios, sever damage to the transfer case and other truck components will occur
If changing the ring and pinion is too costly, changing the tire size to help with fuel economy or towing could be a cheaper alternative. If saving fuel is the primary focus, using overdrive as much as possible will help to save fuel. If towing, using the trucks lower gears is another alternative.
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