Sunday, February 9, 2014

GMC Pickups 101: What’s the Differential?


What an axle ratio means and why picking the right one is important

Editor’s Note: For more than 100 years, GMC pickups have hauled the loads that have helped keep America running. Beneath them have been rear axles with different gear ratios. What do numbers like 4.10:1 mean? This second in an occasional series of “GMC Pickups 101” features explains rear axle ratios.


DETROIT – Technologies like advanced turbodiesel V-8 engines or hydroformed steel frames have advanced the 2013 GMC Sierra to unprecedented levels of capability for fullsize pickups. In just the decade since the 2003 model debuted, maximum towing capacity for a Sierra HD has risen 43 percent, from 16,100 pounds to an industry-leading 23,100 pounds.

Getting that towing force from the engine to each rear wheel is the task of a set of gears located in the center of the truck’s rear axle known as the differential. The ratio of the sizes of those gears is represented by the diameter of the gear that drives the wheels in relation to the gear from the driveshaft.  So, a 3.08:1 ratio indicates the drive gear has 3.08 times as many teeth as the gear on the driveshaft.

Using different diameter gears within the differential affects both towing ability and fuel efficiency. Since pickup owners have varying needs and preferences, GMC addresses different requirements by offering various rear axle gear ratios.

“A numerically lower axle ratio keeps engine speeds lower for better fuel economy, while higher ratios generally yield higher towing capacities and quicker launches from a stop,” said Greg Martuch, energy and powertrain engineer for the GMC Sierra.

Four different ratios are available across the Sierra 1500 ½-ton pickup lineup: from 3.08:1 to 3.73:1. Available ratios vary by cab type, engine and drive type. Sierra HD, including ¾-ton and 1-ton models, uses rear axle ratios of 3.73:1 and 4.10:1.

Each ratio is decided after months and years of testing factors such as acceleration from a stop; performance on a grade; transmission behavior; fuel economy; towing, and thermal management. Keeping engine speeds down allows a vehicle’s cooling to be more effective.

How much of a difference does the choice of axle make? A 2013 Sierra 2500HD with a 6.0L V-8 and four-wheel drive can tow a maximum of 9,900 pounds with a 3.73:1 ratio or 14,400 pounds with a 4.10:1 ratio. Martuch estimates the fuel economy difference at highway speeds would be around 0.2-0.3 mpg between those two ratios, though reduced engine noise is also a benefit of maintaining lower engine speeds with the more-efficient ratio.

Martuch recommends customers consider their everyday needs. “The higher the combined weight of a truck and trailer, the harder the vehicle needs to work. If a customer is going to tow a small boat just a few times a year, a Sierra 1500 with a lower ratio would capably handle the occasional towing duty while delivering better efficiency the other 95 percent of the time. But for someone towing construction equipment every day, I would definitely recommend an HD pickup with a 4.10 axle and external engine and transmission oil coolers.”

GMC has manufactured trucks since 1902, and is one of the industry's healthiest brands. Innovation and engineering excellence is built into all GMC vehicles and the brand is evolving to offer more fuel-efficient trucks and crossovers, including the Terrain small SUV and Acadia crossover.  GMC is the only manufacturer to offer three full-size hybrid trucks with the Yukon, Yukon Denali SUVs and the Sierra pickup. The Sierra Heavy Duty pickups are the most capable and powerful trucks in the market. Details on all GMC models are available at http://www.gmc.com/, on Twitter at @thisisgmc or at http://www.facebook.com/gmc.
  

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