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AXLE RATIOS MADE SIMPLE
Definition: The axle ratio is the relationship
between driveshaft revolutions and axle
revolutions. It’s usually expressed as a numerical
ratio, such as 3.00:1. The higher the axle ratio, the
more engine revolutions are applied to move the
load. The concept is similar to that of leverage. The
leverage comes at a cost, however, by decreasing
fuel economy and top-end speed. For optimum
performance with good fuel economy, a truck
requires the proper ratio on its driving axle. Dodge
Trucks are available with a wide range of rear axle
choices, because no single ratio can meet varying
factors such as engine output, tire size, load and
terrain. All these factors must be considered when
selecting an axle ratio to best meet your needs.
Axle ratios: A closer look
The axle ratio is critical in the operation of the
transmission/driveshaft/rear axle system that
transmits engine torque to the driving wheels
and lets the engine develop sufficient revolutions
per minute (rpm) to run efciently. With too few
revolutions per minute, an engine pulling a load
on a grade is likely to “lug” and overheat. Imagine
your vehicle trying to accelerate from low speed
in its highest gear. With too many rpm, an engine
will run noisily, get poor fuel economy and wear
faster. The best rear axle ratio for the intended
application lets the engine operate at its optimum
power range, typically between peak torque and
peak horsepower. This optimum range is different
for each engine and can be determined from the
engine’s torque/rpm chart.
A low numerical rear axle ratio, such as 3.21:1,
is also referred to as a high ratio, because the
road speed is relatively fast for any given engine
speed. The characteristics of this ratio are lower
engine rpm, reduced power output, and higher fuel
economy. A high numerical rear axle ratio, such as
4.10:1, is also referred to as a low ratio, because
the road speed is slow relative to engine speed. The
characteristics of this ratio are higher engine rpm,
more easily available power output, and lower fuel
economy.
Spec the ratio to the need
How will your truck be used? How much will it haul
or tow? Will it be used on steep grades, on at
highways, or both? How important are fuel economy
and acceleration? These are the questions you
should ask yourself when determining the proper
axle ratio for your needs.
The rear axle ratio is
determined by dividing
the number of teeth on the ring gear (A)
by the number of teeth on the pinion gear (B)
inside the differential. With a ratio of 4.00:1, the
pinion gear on the driveshaft makes four rotations
for each rotation of the larger differential ring gear
on the rear axle driving shaft.
Dodge Truck Axle Ratios
High Numerical Midrange Low Numerical
(“Low Ratio”) (“High Ratio”)
4.88 4.56 4.44 4.10 3.92 3.73 3.55 3.21
Steep Grades Varied Terrain Flat Terrain
Maximum Towing Moderate Towing Light Towing
Maximum Loads Moderate Loads Light Loads
More Pulling Power, Faster Acceleration, Lower Fuel
Economy, Higher Engine rpm/Noise, More Engine Fan
Cooling, Less Prone to “Lug,” Slower Top Road Speed
Less Pulling Power, Slower Acceleration, Higher Fuel
Economy, Lower Engine rpm/Noise, Less Engine Fan
Cooling, More Prone to “Lug,” Faster Top Road Speed