The Medium-Duty Powertrain Puzzle: Putting Together the Pieces

Have you ever taken delivery of a medium-duty truck that wasn’t up to the job? On paper, the truck meets your payload requirements, but for whatever reason, it just seems underpowered, especially when operating near maximum load capacity.

If so, you’re not alone. Even the most seasoned fleet managers learned this lesson the hard way at some point in their careers. Merely knowing your application requires a 26,000-lb. gross vehicle weight rating (GVWR) chassis is not enough. It’s necessary to spec a powertrain that adequately propels the load and efficiently transports it to where it needs it to go. Have you put together the right powertrain package to get the job done?

Pieces to the Puzzle

The powertrain consists of three components — engine, transmission, and drive/rear-axle ratio. The objective is to fit the right pieces together to perform the tasks assigned for the truck, whether pickup and delivery, dump application, on-highway transport, and so forth.

• Engine. Diesel or gas (if available)? If diesel, what horsepower and torque ratings should be spec’ed?
• Transmission. Manual or automatic? (Manual transmission is limited by class and manufacturer.) If opting for the automatic transmission, which size and capacity will match the engine selection?
• Drive/Rear-axle ratio. What drive-axle ratio best fits the application? And is it compatible with the engine and transmission selected? If running trucks on steep-graded hills, will the selected rear-axle ratio match the job description?

How do you sift through the myriad options available on all three components to determine what best fits your application?

Defining Your Parameters

If you put together puzzles as a child, you may recall your parents or older siblings teaching you to start with the edge and border pieces first; then fitting the other pieces together comes much easier.

Think about starting the powertrain selection in the same way. What parameters should be set to ensure the correct engine, transmission, and rear-axle ratio specs fall into place?

The following seven factors define powertrain requirements:

1. Purpose.

Why do you need this truck? Are you looking to haul stone, debris, pieces of pipe, pallets of sod, medical supplies, or bulk paper products?

2. Truck Class.

If, for instance, you plan to haul six pallets of stone on a flat bed, what payload capacity do you need? Suppose, for example, each pallet weighs 1,800 lbs. Your net payload requirement is 10,800 lbs., plus the truck’s body weight. If the body weighs 2,000 lbs., gross payload is 12,800 lbs.

This information is important to consider because it narrows down which class truck best fits your needs. And truck class determines the number and type of engine and transmission combinations available.

Using 9,000 lbs. as a generic weight number for the chassis, total chassis and payload equals approximately 21,800 lbs. gross vehicle weight (GVW), therefore narrowing the options to Class 6 powertrain combinations.

The following is a truck classification breakdown by GVWR.

1. Class 4: 14,001-16,000 lbs.
2. Class 5: 16,001-19,500 lbs.
3. Class 6: 19,501-26,000 lbs.
4. Class 7: 26,001-33,000 lbs.

3. Trailering Requirements.

Are you towing a trailer? What is the total weight of the trailer and its payload? This equation addresses gross combination weight rating (GCWR) requirements — the maximum allowable weight of the truck and payload combined with the trailer and its contents.

4. Type of driving.

Will the truck be driven inner-city for stop-and-go deliveries? Will it operate on highways at full speed? Or, will it be used for offroad applications running at slower speeds? Answers to these questions impact all three powertrain component decisions.

5. Annual Miles.

Use this information to confirm whether to spec a diesel or gas engine (if available), depending on how long it takes to recoup the higher initial cost of diesel through proposed fuel economy savings.

6. Terrain/ Geography.

Will the truck be operated on flat roads? Or, will it be used to pull heavy loads over hills? The application sets the parameters for choosing the right engine torque ratings and rear-axle ratios for best performance regarding terrain.

7. Driver Profile.

Who will drive the truck? Will it be assigned to a specific driver or will the truck be spec’d so that anyone can drive it? If the truck has one driver, what is that driver’s skill-set? Can he or she operate proficiently with a manual transmission? This answer helps determine transmission selection.

Engine Selection

Based on the information gathered above, what class truck is required?

• Class 4 & 5: If the truck is a Class 4 or 5 (up to 19,500 lbs. GVWR), engine selection is relatively simple. In many cases, diesel is the only option. Some manufacturers, such as Ford (F-450 and F-550), GM (C-Series and WSeries), and Isuzu (N-Series and H-Series), offer both gasoline and diesel engines.

How do you decide between gas and diesel for a Class 4 or 5 truck? While diesel offers better fuel efficiency and engine longevity, gas engines have a much lower upfront cost — a difference ranging from $6,500 to $8,000 or more, depending on make and model.

Therefore, the key deciding factor should be the total miles the truck will be driven annually and whether the driver will log enough miles to recoup the higher investment in the diesel engine in a reasonable amount of time.

A general rule of thumb is the 25,000- to the 30,000-mile threshold. If you project exceeding 30,000 miles, diesel makes sense. If less, then a gas engine may be more cost-effective.

However, run the numbers to confirm what works best for your fleet. Contact the manufacturer’s representative for approximate fuel economy numbers for both gas and diesel to plug in the calculations.

• Class 6 & 7: If spec’ing a Class 6 or 7 truck, engine selection gets more complicated. For example, the Isuzu Diesel 6H 7.8L in the Chevrolet Kodiak C-7500 offers five different horsepower and torque options, ranging from 215 hp and 560 lb.-ft. torque to 300 hp and 860 lb.-ft. torque, with 230 hp and 260 hp in between. The approximate cost difference between the lowest and highest horsepower ratings, when factoring in a heavier transmission required for the 300-hp engine, runs as high as $8,500. The variance in engine horsepower/torque availability and cost is common across all Class 6 and 7 truck OEMs. 

Matching the Right Horsepower & Torque Ratings

How do you choose among the various horsepower and torque options? Under-spec’ing the engine may lead to performance and potential premature maintenance issues. Over-spec’ing may result in paying a much higher cost per unit without a corresponding return on investment. How do you strike the right balance?

At this stage, involve the equipment upfitter and truck OEM representative for their counsel. They often have experience working with fleets in your industry and can offer specific recommendations.

The following general principles based on common applications can help serve as a guide.

Higher horsepower and torque applications:

• Over-the-road with a heavy load, such as a towing and recovery vehicle.
• Hilly, mountainous region.
• Over-the-road pulling a heavy trailer

Lower to mid-range horsepower and torque applications:

• Pick up and delivery in which the truck does not often reach full highway speed.
• Slower speed applications such as an asphalt spreader, a dump body, mechanic’s crane, or service body truck.
• Over-the-road, on flat terrain, with a lighter load.

​Transmission Selection Selection

After selecting the right engine based on your parameters, how do you decide on the right transmission?

Class 4 & 5.

With this truck size, you’re typically choosing between two options: one manual and one automatic transmission. Depending on make and model, an automatic may be the only transmission available.

How do you decide which is best for your fleet? Start by evaluating who will drive the truck.

Manual Transmission: If only experienced, skilled drivers will operate the truck, a manual transmission may be a better fit. Here’s why:

• Better fuel efficiency: That is, if the driver is skilled and can shift gears proficiently.
• Lower initial cost. The manual transmission for medium duties ranges from $2,000-$4,000 less than a corresponding automatic transmission, depending on the truck’s make, model, and GVWR.
• Greater shift control. This can be an advantage for off-road applications and driving on hills and bends. Once again, driver skill is important here.

Automatic Transmission If a truck that just about any employee can drive is necessary, an automatic may be worth the extra investment. The following are the benefits of automatic transmissions:

• Enhanced safety. Drivers can keep their eyes on the road and hands on the steering wheel. There’s no need to worry about the driver inadvertently allowing the truck to roll backward when at a stoplight on a hill.
• Expanded driver pool. More driver options are offered because the automatic transmission doesn’t require as skilled a driver. The more skill the truck requires to operate, the higher the cost to find qualified drivers.
• Less driver effort. The driver doesn’t have to constantly push in with one foot and actively shift gears with one hand. The automatic transmission does the work.

Class 6 & 7.

This is where transmission selection gets more complicated because multiple manual and automatic transmission options are available. Consider the following points:

• Manual or automatic? Use the same guidelines as above for Class 4 and 5 trucks.
• Compatibility with engine horsepower and torque ratings. Whether automatic or manual, select only the transmission options that match the chosen engine’s horsepower and torque ratings.
• GCWR requirements. If trailering is important to the application, which transmission option provides the most capacity? For instance, two Allison automatic transmissions are compatible with the same 660-lb.-ft. torque engine. However, the 2500 Allison offers only 33,000 lbs. GCWR, while the 3000 Allison bumps it up to 60,000 lbs. GCWR.
• If automatic, wide ratio or close ratio? If the application is primarily over-the-road at highway speeds on relatively flat terrain, a close ratio transmission will suffice. Opt for a wide ratio when requiring greater low-end reduction to run trucks for both on- and off-highway use and pick up and delivery with numerous starts and stops.

Rear-Axle Ratio Selection

As the third piece of the powertrain puzzle, the rear-axle ratio represents the relationship between driveshaft revolutions (driven by the transmission) and rear-axle revolutions. In medium-duty trucks, this gear ratio ranges from as low as 2.69:1 to as high as 7.17:1, depending on the truck class, make and model, and rear-axle capacity.

• Class 4 & 5: As with engine and transmissions, axle ratio options are limited in this size truck. For instance, an Isuzu NPR cab-over rated at 14,500 lbs. GVWR with diesel and automatic offers one ratio: 5.125:1. The conventional cab Ford F-550 provides just a handful of ratios to choose from, ranging from 4.30:1 to 4.88:1 with the diesel.
• Class 6 & 7: In contrast, this truck size presents as many as 30 or more ratio options. How do you decide which is the best fit for you?

Rear-Axle Ratio Spec Guidelines

The following general guidelines should help point you in the right direction:

• High numerical ratios. This is for maximum towing and payloads and on hilly terrain with steep grades.
• Mid-range numerical ratios. Spec this ratio when the truck requires flexibility for operating on varied terrain with moderate towing and payloads.
• Low numerical ratios. This makes sense for flat terrain, lighter loads, and running at consistent highway speeds.
• Power vs. Efficiency. The rule of thumb is the higher the numerical ratio, the greater the pulling power, but at the expense of fuel economy. The inverse often holds true. Reducing the numerical ratio will improve fuel economy, but sacrifice pulling power. Find the ratio that best fits the balance you’re trying to achieve.

Putting it All Together

As author Steven Covey puts it in his best-selling book 7 Habits of Highly Effective People, begin with the end in mind. Start with a clear idea up front of how the completed powertrain “puzzle” should function. Do you need the truck to perform on steep grades, carrying maximum loads, while pulling a loaded trailer? Will the truck be operated at consistent highway speeds on flat terrain? Or something in between? With the end-use in sight, it is much easier to match the right engine to the right transmission to the right rear-axle ratio to perform the right job.