Carrying your vocational load means balancing the right axle ratio with the right axle weight rating. Just as advanced engine technology and deeper integration have enabled faster axle ratios on-highway, so too have those engine efficiencies translated to the vocational world.
“Today’s engines are more advanced and efficient than ever,” said Kelly Gedert, director of product marketing Freightliner and Detroit components, “so it’s also important to review axle ratios and HP/torque ratings when buying a new truck.
“Each engine OEM has unique power and torque curves that need to be complemented by the correct axle ratio,” she continued. “The proper axle ratio will ensure proper startability, gradeability and top speed, while maintaining efficient operation of the engine and after treatment.”
“Many times, vocational trucks are ordered with the same rear axle ratio as the customer’s previous truck because it worked fine before, but modern engines make much more torque at lower RPM and don’t require the high engine speeds to do the job as they did in the past,” said Ryan Breezee, vocational marketing manager for Kenworth. “With torque at low RPM, there is an opportunity to select a faster gear ratio [lower numerically] and still get the same or better performance—while increasing fuel economy and reducing unnecessary wear and tear on the powertrain.”
Tony Sablar, vocational marketing manager for Peterbilt, noted that “the rear axle ratio is the most common method for changing the performance.”
This includes limiting the on-highway speed, added Bruce Vasbinder, director of severe service product marketing for Navistar. “Additionally, an incorrect ratio might not be sufficient to do the work required of the application,” he said.
The fact of the matter, as Dana’s Steve Slesinski, director of global product planning for Dana noted, is that “spec’ing the correct ratio to provide the right level of startability and gradeability is as crucial as spec’ing the appropriate drive axle to support the vehicle’s gross axle and combination weight ratings.”
Working with your OEM and suppliers is the best way to gauge the right ratio for you.