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.”
