Moses Ludel’s 4WD Mechanix Magazine – How-to: Choosing an Axle Gear Ratio
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 mile).
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 mile.
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 choice.
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 range.
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.*
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 mph).
See this HD video on the use of the Cummins PowerSpec calculator to help select axle gearing.
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.