Optimizing Gear Ratios for Work Trucks
Have you ever found yourself confused when selecting a rear axle ratio on a vocational spec? This month, we are focusing on how to optimize your gear ratios for work trucks.
Before selecting a rear axle ratio, make sure you have a transmission that is best suited for the job and completely understand what the customer is expecting the truck to do.
When choosing a rear axle ratio, the three main factors to consider are cruise RPM, startability, and gradability. Make sure that the information put into PremierSpec such as GVWR, max grade, and road class is accurate! If the inputted information isn’t accurate, the output information won’t be of any use.
Note: If any drivetrain components such as engine ratings or transmissions are added via the NPO screen, the rear axle ratio workscreen will not be updated. It will populate information based on the published options that are selected. Even if NPO/OAR codes are replacing the published content.
Selecting a transmission can be a daunting task in itself. Manual transmission are the tried and true choice and still allow the driver to have the most control. However, they require an experienced driver to get the most of the transmission. Automated and automatic transmission are also perfectly viable choices and lower the strain on drivers by giving them one less thing to think about.
The more gears a transmission has, the easier it is to keep the engine in its peak torque range and keep the momentum of the truck going. This is why most heavy haul applications will use 18 speed transmissions. While selecting a transmission with less gears may need a higher torque engine to compensate for the lack of gear selection. Another thing to note is that a transmission designed with low (numerically higher) ratios in the first several gears can be used with a higher (numerically lower) rear axle ratio and still maintain performance when starting and accelerating, but not sacrifice fuel economy while at highway speeds. This concept is called overall ratio coverage. Transmissions that include low starting ratios include Eaton 8LL, 18 speed manuals, The PACCAR Automated, VCS Ultrashifts, and Allison 3500/4500RDS transmissions.
If the truck needs to maintain very low speeds for applications such as curbing, make sure to spec a transmission that has a deep first gear that is specifically designed for extremely low speed movement. Examples of these transmissions are the Allison 4700 as well as the Eaton 9LL (both manual and automated).
The PACCAR transmission is an excellent choice for select vocational applications that will spend the majority of the time on road. The PACCAR transmission offers the best overall gear ratio coverage, which means excellent startability while still allowing for a low RPM cruising speed for fuel economy.
Note: If spec’ing an Allison transmission, always run an ISCAAN to optimize the configuration. The ISCAAN program can be run by your local Allison rep or can be requested from our sales engineering group (firstname.lastname@example.org)
Cruise RPM and Optimization Goals
Cruise RPM is how fast the engine is turning when the truck is at highway speeds. The typical cruise speed can be set in PremierSpec. This is the maximum typical highway speed that the truck will see (not the overall average speed of the truck). PremierSpec will recommend engine speeds at cruise based on selected optimization goals. For light vocational applications (<60,000lbs GCW) that will see a lot of time at highway speeds, a balanced optimization goal may be the best option. For heavy vocational applications, trucks which will see very limited time at highway speeds, or heavy haul applications you should consider a performance optimization goal. A balanced optimization goal will recommend 1,350-1,550 RPM cruising engine speed while a performance optimization goal will recommend 1,450-1,650 RPM for MX engines (PX-9 recommended RPM ranges will be much higher). Keep in mind that modern engines produce peak torque at much lower RPMs than older engines, so just because a rear axle ratio worked well for a customer 15 years ago doesn’t mean that it will be the best choice for their new truck.
These are general guidelines and there are many exceptions that come into play. Even though a vocational truck has a GVWR of only 56,000lbs, it may still may benefit from a lower (numerically higher) rear axle ratio and a performance optimization goal if it will spend a lot of time on hilly terrain. The opposite may be true with a truck that has a 70,000lb GVWR which will be running on flat ground. This spec may be better served with a balanced optimization goal and a higher (numerically lower) gear ratio.
This is the maximum grade that the truck can start on when fully loaded. The minimum required startability is a calculation that takes into account application, road class, and performance goals. As a rule of thumb, we recommend a 20% startability rating in 1st gear, and 16% in 2nd gear. Typically, this isn’t an issue if the rear axle ratio is set so the cruise RPM and gradability is acceptable. In order to view the startability chart, select Speed & Grade – Startability from the reports section in Premierspec. If startability targets aren’t met, a lower rear axle ratio (numerically higher), a higher engine HP/torque rating, or a combination of both may be needed.
Gradability is the max grade that the truck can sustain when fully loaded. You can view the full gradability chart which shows the max gradability in each gear when the engine is at peak torque by selecting the Speed & Grade – Gradability from the reports section in PremiersSpec. You can also view a summary of the spec’s gradability in the top two gears in the rear axle ratio work screen. Just like startability, the minimum required startability is based on several factors. However, a good general recommendation for vocational applications is to make sure the truck will be able to sustain a 1.8% grade in top gear at minimum. The spec shown in the screenshots below can sustain a 2.0% grade in top gear and a 3.3% grade in second to top gear (9th gear in this case). If gradability targets aren’t met, a lower rear axle ratio (numerically higher), a higher engine HP/torque rating, or a combination of both may be needed.
MX Rear Axle Workscreen
If spec’ing an MX engine, there will be recommended gear ratios highlighted in green in the rear axle workscreen once the optimization goal and typical cruise speed has been selected. The rear axle ratios highlighted in green meet the requirements of the recommended cruise RPM, minimum startability, and minimum gradability.
If no rear axle ratios are highlighted in green, check the messaging box in the rear axle workscreen to review what criteria isn’t met.
If a Cummins engine is spec’d, please refer to PowerSpec for gearing optimization. The PowerSpec program can be found using the following link. https://www.cummins.com/parts-and-service/digital-products-and-services/powerspec
Note: If working with a spec that will be hauling 140,000lbs or more, your spec may require 2 speed rear axles or an auxiliary transmission. it is always a good idea to consult sales engineering at email@example.com to get their recommendations.