10 Factors to Consider when Spec’ing Drive-Axle Ratios for Medium-Duty Trucks
There is a wide range of drive-axle ratios for medium-duty trucks. Considering drive-axle
ratio impacts a truck’s top-end speed, load-pulling ability, and overall fuel economy,
fleet managers must choose carefully.
July 2011, Work Truck - Feature
The gear ratio selected for the drive axle impacts the truck's available top-end speed, ability to haul or pull heavy loads, and overall fuel economy.
|At a Glance:
Spec'ing the correct drive-axle ratio can impact performance, speed, and fuel economy:
- A higher ratio gives greater power, but lower top-end speed and fuel economy.
- A lower ratio results in less pulling power, but improved top-end speed and fuel economy.
When it comes to spec'ing light-duty trucks and vans, the process of selecting a drive-axle ratio (also known as rear-axle ratio in rear-wheel-drive trucks) is relatively simple - there are only two to three ratios to choose from.
Medium duties, however, especially those in the 19,500-lb. to 26,000-lb. gross vehicle weight rating (GVWR) range, complicate the process. There are often more than a dozen options to choose from, with drive-axle ratios ranging from as low as 2.69:1 to as high as 7.17:1 - with 10-12 ratios in between - depending on the truck class, make and model, and rear-axle capacity.
Considering that the drive-axle ratio impacts a truck's available top-end speed, ability to pull heavy loads, and overall fuel economy, fleet managers must choose carefully.
However, with numerous axle ratio options available with medium-duty trucks, how does a fleet manager determine which is right for the application?
Understanding Drive-Axle Ratios
The starting point is to understand what the drive-axle ratio means and how it affects truck performance.
The drive-axle ratio represents the relationship between driveshaft revolutions (driven by the transmission) and drive-axle revolutions. Usually a whole number and decimal fraction, the drive-axle ratio is a comparison of the number of gear teeth on the ring gear of the rear axle and the pinion gear on the driveshaft.
For example, a 4.11:1 ratio means there are 4.11 teeth on the axle's ring gear for each tooth on the driveshaft's pinion gear. Or, put another way, the driveshaft must turn 4.11 times to turn the rear wheels one full revolution.
What does this ratio mean in practical, performance terms? Here's the typical rule of thumb: The higher the numerical ratio, the slower the gear will be. This higher ratio gives a truck greater pulling power, but since the engine must work harder to spin the driveshaft more times for each turn of the rear wheels, top-end speed and fuel economy are sacrificed.
The inverse also holds true. When numerical ratio is lowered, available top-end speed is increased. Since the engine doesn't have to work as hard to turn the wheels faster, fuel economy improves. The downside is that pulling power diminishes.
Matching the Axle Ratio with Vehicle Application
How can a fleet manager strike the right balance between speed, pulling power, and fuel economy to best fit a truck's application? Ken Gilles, truck operations manager for GE Capital Fleet Services, recommends fleet managers consider these 10 factors when spec'ing drive-axle ratios:
1 Speed. What is the desired maximum road speed for this truck? What is the average speed expected?
"Look at where the truck spends the most time from a speed perspective. What does that mean in terms of where my engine RPM [revolutions per minute] is at in top gear? How does that translate in rear axle ratio?" Gilles said. "What we're after is to ensure the torque and horsepower curves are at the right spot for that particular engine so the truck is operated predominantly in its optimal operating conditions - e.g., is the truck spending most of its time on urban roads at 40-60 mph? Try to gear the rear axle to run the engine in that RPM range."
2 Gradeability. "This means the ability to maintain specific road speed on X-percentage grade [incline]," Gilles explained. For example, if the truck is in Texas, the biggest thing you have to deal with is a freeway overpass grade. That's easy, but if you're operating the truck in constant steep-grade conditions in the Appalachian Mountains, that's a different matter. You need to gear [the drive axle] for a lower speed to perform in mountainous terrain."
3 Load. Is the truck's load increasing, diminishing, or constant? What percentage of time is spent at what percentage GVWR?
"If you have a diminishing load, that takes us down the path to allow for a slightly faster gear in the truck since it's going to get lighter during the day," Gilles said. "If it is constant load - like a service truck that has a crane mounted on it, weighed down with tools, compressor, and so forth - then it's always at its max or close to its max from a GVW standpoint. If the truck is operating like that, then you need to back off on the maximum road speed availability and look more at where it's going to deliver the best start-ability and fuel economy under the constant load."
4 Towing. What is the trailer and load weight? What is the size and length of the trailer? What percentage of time will the truck be towing?
"Depending on what percentage of time you're operating the truck in a tow scenario, you might need to gear the axle slightly lower from a handling standpoint," Gilles advised. "If you're pulling a trailer as much as 20 percent of the time, that is enough to perhaps gear it slightly lower and give up some of that maximum road speed for the truck to handle a greater load."