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Choosing the CorrectAxle Gear Ratio for Oversized
The Ram 3500
4WD came equipped with a Cummins 5.9L diesel. Original gearing was 3.73:1 with rated 31.4" diameter
tires (officially 658 revs per mile for the Dodge Ram truck). The magazine's makeover of the
truck included tires rated 34.6" diameter (BFG claimed 601 revs per
As is often
the case, the new wheel rim diameter was at the narrower, yet still acceptable range for the tires
(9-inch rim width: 9 to 12 inches acceptable). After careful testing, it was determined that the
tires are a true 34.65" diameter in this application. This translates to 582 revolutions per
We had the option of simply restoring
the axle ratio. At the new tire diameter, 4.10:1 would provide a similar engine cruise
rpm to the original tires with 3.73:1 gears. Trailering plans and 1,100 pounds of upgrade accessories
and auxiliary fuel changed the equation, however. 4.56:1 became the
A lingering consideration was whether the 5.9L Cummins diesel
would work well with lower (numerically higher) gearing. We ran the 4.56:1 axle gearing and actual
34.65" tire diameter (582 revs per mile) through the Cummins PowerSpec calculator. Based on both
light load and trailering, a balance between economy and reasonable performance,
the targeted cruise speeds were in the 60-70 miles-per-hour
Prior to the makeover and suspension lift, the Ram had provided
exceptional fuel efficiency (23.9 mpg consistently, 25 mpg at peak), running in the 1600-1900 rpm range
at cruise. The OE axle ratio was 3.73:1...4.56:1 gears with the new BFG 34.65" tires would be like
4.10:1 gears with the truck's original tire diameter.*
*Note: It was helpful to know that 4.10:1
was actually a factory gear ratio option.
With trailering plans and our willingness to work with a 65-68 mph
cruise speed target, we opted for the 4.56:1 gearing. This would place the diesel engine at 1,984
rpm for 65 mph and 2,075 rpm at 68 mph.
4.56:1 gearing has the 5.9L engine well
up the horsepower curve and within its peak torque range. Peak torque arrives at 1,600
rpm, the key to diesel performance.
Below is the rpm result for various cruise
speeds, computed with the 0.69:1 overdrive fourth gear (48RE automatic transmission), true
34.65" tires (582 revolutions per mile) and 4.56:1 gears:
After the 4.56:1 gear change, initial testing indicates an
unloaded cruise speed of 65 mph for optimal fuel
efficiency.This works well with our plans for the truck. 4.56:1 with the 34.65" tires is close to
4.10:1 gears with the OEM 31.4" diameter tires (2,017 rpm at 65
See this HD video on the use of
the Cummins PowerSpec calculator to help select axle gearing.
(If you have enough bandwidth,
open the video at HD 720P and full-screen mode!)
For trailer pulling, all this means less load on the entire
powertrain and improved compression braking on grades—plus optimal fuel
efficiency.Pulling a trailer weighing approximately 7,500 pounds—without taxing the Ram's powertrain
and brakes—we will still get our targeted fuel efficiency.
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