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GMC Sierra Axle Gearing Explained

By:  Connor MC  / Sep 25 2019
GMC Sierra Axle Gearing Explained

Unlike chugging an energy drink or sniffing bath salts (both short term solutions), changing the rear end ratio of your Sierra is a long term method to revitalize and reenergize a struggling truck. If you find your Sierra is straining to tow and/or haul, or have noticed a big dip in performance after upgrading to a large wheel/tire package, swapping out the factory gears is likely the kick in the pants that your truck needs.

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Changing out your gears changes where your Sierra makes its power. Swapping to a gear that brings your power further down your RPMs will assist in towing, but the opposite nets you top end speed for track trucks.

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Why is Gear Ratio Important?

Simply put, the differential gear ratio is the ratio between the number of revolutions of the input shaft (the driveshaft) and the number of revolutions of the wheel. In the case of an axle ratio with a value of 3.73, this means the driveshaft and pinion gear rotates 3.73 times to complete one revolution of the wheel.

One aspect of differential gears that is sometimes confusing is the laymen terms thrown about to describe them. Changing to a lower ratio gear actually means selecting a ring and pinion with a higher numeric ratio. I.E. a 4.10 gear set is numerically higher but is described verbally as a lower gear. A lower gear favors acceleration off the line and low end torque at the expense of increased engine RPM for any given speed, as the driveshaft has to complete more revolutions just to turn the wheel once. A high gear provides high speed at lower RPM but lacks torque off the line. In the world of Sierra half-ton pickup trucks, lower gears are favored which is completely logical given the utilitarian duty that trucks are expected to perform. Towing ability – very important; top speed – not so much.

Why Re-Gear My Sierra?

Having a suitable differential gear for the task at hand is paramount to achieving any level of respectable performance and the specific ratio of the gear dictates just how much pep per step your Sierra will have. Regearing a Sierra is often done when the stock ratio is too high for effective towing, a scenario signified by the transmission constantly downshifting in order to maintain speed or an inordinate amount of throttle input just to get moving. If you look at any GMC towing guide, you’ll notice that the highest tow capacity is always given to the Sierra with the lowest gear ratio. This is due to the extra torque multiplication a low axle ratio provides.

Another scenario that often requires regearing a Sierra is after installation of a large set of wheels and tires. Running a big wheel and tire combo is great for off-road traction and capability, but puts a significantly larger load on the engine (at all speeds) due to the increased mass and diameter. This can lead to very sluggish performance with the engine bogging down and poor throttle control when presented with a high rolling resistance (i.e. overcoming a trail obstacle). The axle ratio GMC installs in a Sierra is picked specifically to deliver good all around performance with a certain tire height. Changing the tire height by going bigger changes the effective axle ratio and causes the stock gearing to act like a higher gear – reducing low end torque.

What Gear Ratio Do I Have?

Determining the gear ratio your Sierra was equipped with from the factory is very easy. You can call or visit a dealer with the VIN number and they will be able to look up the build sheet for you, or alternatively (and much quicker) is to check the RPO codes.

The RPO codes can be found printed on a sticker on a door jam or in the glove compartment. Once you have located the sticker with all the codes, look up an RPO master list from the internet and use that to decode the sticker (there are many RPO codes, and they can change by year, thus posting a list here is not practical).

2007-2009 Sierras: 3.23, 3.42, 3.73, and 4.10.

2010-2018 Sierras: 3.08, 3.23, 3.42, and 3.73.

How Do I Pick a New Ratio?

If you have installed larger tires on your Sierra and just want to know what is the ideal new axle ratio needed in order to return to factory-like performance, there are plenty of online calculators around that will help you figure this out. Original tire size, original axle ratio, and new tire size are the only variables you need in order to calculate a new ratio that is better suited for the larger tires. Of course, keep in mind this will only maintain the factory performance levels. If you seek increased low end performance over the factory setup, then you will need to select a lower gear than as calculated.

To help with this, many aftermarket manufacturers have developed a chart system that shows tire size, engine RPM, and vehicle speed using a certain axle ratio, with select ratios highlighted to show optimum towing or optimum economy.

For towing applications, if your factory gear is not the lowest option available (i.e: you have 3.23 and 3.73 was available for that same year) look at the GMC tow chart and see how your Sierra compares to one with a different ratio. Many factors go into determining the overall tow capacity, such as transmission, braking ability, axle weight rating etc, and thus it is not necessarily safe to say you can increase your tow capacity by changing the gears, but rather you can certainly make towing easier at your current capacity. Additionally, Sierra enthusiast forums have great real-world knowledge on gear selection. It seems the consensus that a 3.73 or a 4.10 ring and pinion are the optimal choices for heavy duty towing. Having said that, please understand not all situations are created equal, and by no means are these the only ratios that should be considered.

One other important thing to keep in mind is a four-wheel drive Sierra will need to change the gears both in the front and rear. You also need to check the RPO codes to see what size differential your Sierra has. It can be 8.5” or 9.5” depending on year and option level.

New Gears and Fuel Economy

Last but not least is the subject of fuel economy. People looking to regear their Sierra pickup are almost always looking to go one way – down to a lower gear. As mentioned above, a lower final drive gear will induce higher engine RPM for any given speed, which might then prompt the thought of fuel economy. It would seem logical that an engine turning at higher RPM consumes more fuel, and thus regearing your Sierra with a lower gear could lead to decreased fuel economy. While this is certainly plausible, real world driving has more or less shown that fuel economy is more or less negligible within the scope of combined city and highway driving. It is impossible to say outright if there will or will not be a change in MPG, but realistically speaking it isn’t something that should be a major factor.

Fitment includes: 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, Classic, NevadaEdition, SL, Z71, Denali, SLE, SLT