Kenworth News

Spec’ing for Fuel Economy

One of the most important factors for over-the-road customers is fuel economy. Even a 1%-2% increase in fuel economy can add up quick. Especially since there is a lot that can be done to a spec to increase fuel economy without increasing the cost of the truck!

Also, keep in mind that you are considered the consultant when putting together a spec for a customer. In many cases, customers are relying on you to build a spec that will work well for their goals and application. If the truck doesn’t perform, the chances of getting another sale can be slim.

This article will go through some of the main considerations when spec’ing for fuel economy. Also, make sure the customer realizes that some options that maximize fuel economy can also compromise performance. This is why we always recommend going over each option with the customer to make sure they understand the tradeoffs and if the spec will still meet their goals.

Before we dive into different options, it’s important to know where fuel is being used. The chart below shows a breakdown of where fuel energy is used at a constant/cruising state. From the graph we can see that the most effort should be focused on engine, aerodynamics and tires.

As shown above, we can see that there is a lot of opportunity to improve fuel economy by spec’ing the engine and supporting equipment properly. Just like any machine, diesel engines are not 100% efficient. Some of the fuel’s energy is turned into heat from friction and combustion, and additional losses result from engine-driven accessories such as the fan, power steering pump, alternator and air compressor. Of course it’s necessary to spin these accessories, but the key to increasing fuel economy is to spin them just fast enough to do their job. This is one of the main reasons that we suggest running a lower engine RPM when cruising. A lot of effort was put into the MX engines to make peak torque as low as 900RPM. This allows our trucks to cruise as low as 1,100RPM and still maintain very acceptable performance. The rule of thumb here is that every 100RPM reduction in cruise RPM is worth 1% fuel economy. The cruise RPM can be adjusted by changing gear ratios, and will be covered in more detail in the transmission and rear axle sections.

Another thing to consider when spec’ing an engine is how much horsepower and torque the customer needs. A good tool for determining this is the speed and grade charts in PremierSpec. Make sure to enter accurate weight and cruise speed, or the tool will give you bad results. As they say, garbage in, garbage out. A common rating for a linehaul customer that is carrying 80,000lbs is a MX-13 455/1650. However, you may want to talk to your customer about the 405/1650 rating. The 405 rating has the same amount of torque as the 455 rating, but the torque drops off at a lower RPM. This encourages the driver to keep RPMs lower and increase fuel economy. Keep in mind this might not work for all customers. If the truck will be used on a route with steep grades, a higher HP rating will most likely be needed as well as a numerically higher gear ratio.

Finally, if fuel economy is a major concern, the MX-11 should be considered. The MX-11 not only saves weight over the MX-13, but it also provides a fuel economy improvement. And with a 430/1650 rating available, it’s more than capable of handling most linehaul applications.

The model is the most obvious selection when fuel economy is important to the customer. The T680 provides the most aerodynamic hood design and is the only model available with fairings. Unless your customer plans on going off road or is concerned with ground clearance, we strongly recommend chassis fairings. Chassis fairings alone can reduce aerodynamic drag by 4%! As a rule of thumb, reducing aerodynamic drag by 1% will result in a .5% fuel economy improvement. That being said, chassis fairings can bump fuel economy by 2%. This alone could save your customer over $900 per year if they are driving more than 100,000 miles! The most fuel-efficient chassis fairing configurations are either partial fairings or full fairings. Both full and partial have similar aerodynamic drag factors, however, partial fairings do provide more access to the rear suspension.

We also strongly recommend matching the roof height with the trailer/body height. For example, a truck that is spec’d with a midroof sleeper with a full height van trailer can increase aerodynamic drag up to 18%. That’s a 9% fuel economy loss! This goes for daycabs as well. A roof fairing is recommended for all fuel-economy-conscious daycab specs that are running full-height trailers or bodies.

Another thing to know is that each T680 comes standard with the following aerodynamics aids when applicable:

  • Front air dam under bumper (not available with chrome bumper)
  • Flexible chassis fairing extensions
  • Closeouts for both chassis fairings and daycab roof fairing
  • Flexible side extender trailing edges
  • Wheelwell closeouts

Tires can be one of the more confusing components to select. Luckily, PremierSpec adds a “+” by tires that are recommended for fuel economy. However, we recommend talking to the tire manufactures directly for an in-depth recommendation.

No matter what tire you select, spec’ing the Tire Pressure Monitoring System is highly recommended. While this is an added cost up front, it can easily pay for itself by alerting drivers when tires are underinflated. If a single tire is underinflated by only 10psi, it can cost up to 1% fuel economy.

According to the chart above, the transmission only takes up 1.2% of the total fuel energy when cruising. It may seem like the transmission would be an area of less concern when spec’ing for fuel economy. But the transmission is responsible for keeping the engine RPM in the sweet spot and balancing fuel economy with performance.

The first decision is what type of transmission to select. For on-road, fuel-economy-focused applications, manual or automated transmissions are almost always the transmission of choice. A torque converter automatic transmission has its place, but the friction losses are much higher in automatics, which usually makes them undesirable for over the road applications. (There are some exceptions, of course).

While a manual transmission gives the driver more control, an automated transmission will almost always get better fuel economy than a manual. Even the most experienced and motivated drivers will have difficulty getting the same fuel economy as an automated transmission. This is partly because an automated transmission can be programmed to get the absolute best fuel economy possible. Another reason why automated transmissions typically get better fuel economy is because they can be more flexible with gear splits, while a manual must have more even gear splits to make shifting intuitive for the driver.

As mentioned earlier, the ideal cruising RPM for a fuel-economy-focused spec is 1,100RPM. In order to do this and still maintain an acceptable level of performance, the transmission must have a low enough gear so that the truck can start easily on a hill, but a high enough top gear to keep RPMs down at cruise.  Another trick to maximizing fuel economy is a short step between the transmission’s top gear and the gear one down from top gear. This short step allows the engine to maintain the lowest possible RPM when cruising and then the transmission almost seamlessly shifts down when going up gradual hills to maintain speed.

One reason the PACCAR transmission has become the most popular choice of over-the-road customers is because of its excellent overall gear ratio coverage. The low gears give plenty of reduction to get the truck moving, but the top gears are close together and are high enough to lower engine RPM for the absolute best fuel economy during cruising.

Rear Axle Ratio:
The rear axle and ratio selection is the last piece of the puzzle when determining cruise RPM. A higher rear axle ratio (lower number) will lower engine RPM while the truck is cruising. Selecting a rear axle ratio depends on many things. The factors that play into selecting a gear ratio are the truck’s typical cruising speed, engine, transmission, rear tire size, and of course the customer’s expectations.

Luckily, the rear axle workscreen in PremierSpec makes selecting the rear axle ratio easy. Just input the following information and PremierSpec will recommend a gear ratio.

Typical Cruise Speed: This is the typical speed the truck is going when cruising on the highway in top gear. This is NOT the average speed overall. For example, a common typical operating speed might be 65MPH.

Optimization Goal: This is what is most important to the customer. If they have a laser focus on fuel economy, then select downsped economy. If fuel economy is important, but they still want the truck to feel more responsive, select fuel economy or balanced.

Once these fields are populated, the recommended gear ratios are then highlighted in green. Often there is more than one gear ratio that is highlighted. As mentioned before, the higher (lower numerical) gear ratio will provide better fuel economy, but will sacrifice some performance.

As a good baseline, a PACCAR transmission paired with a 2.64 axle ratio and 11R22.5 tires is a great starting point for a fuel-economy-based spec.

Trailer Gap:
The trailer gap is an easy way to make sure your spec is optimized. The more distance there is between the rear of the cab/sleeper, the more turbulence there is going to be. The general rule is that for every 1” a trailer gap can be reduced, fuel economy will be increased by .1%. While this doesn’t seem like much, it can start adding up quick if the trailer gap is too large. It’s also free to optimize the trailer gap with no negative effects on performance. Just make sure you’re careful there is enough swing clearance between the truck and trailer.

The swing clearance workscreen in PremierSpec can be a very useful tool to dial in trailer gap. However, make sure that the trailer dimensions are correct in the Vocational Dimensions to get the most accurate results. Kenworth recommends a trailer gap of 38-42” for best fuel economy. The gap is measured from the back of cab or sleeper and does not take into account side extenders.

When adjusting trailer gap, keep in mind that moving the fifth wheel will change weight distribution. When the trailer is moved forward to minimize trailer gap, more weight will be put on the front axle. The wheelbase can be shortened to counteract this.

Tip: If you are having trouble minimizing trailer gap without overloading the front axle, try changing from an MX-optimized hood to a standard length hood. The longer hood will push the cab/sleeper back and reduce the trailer gap by 6”.

Tip: If swing clearance fails before getting a trailer gap of 42”, try changing to a frame-mounted tractor kit.

Other Options:
Aside from the options that were mentioned previously, the following options can be spec’d to improve fuel economy. This isn’t an exhaustive list, but it’s a good starting point.

  • Flow-Below wheel covers and rear axle fairings - UP to +2% Fuel Economy
  • Tire Pressure Monitoring – 10PSI underinflation on one tire can reduce fuel economy by 1%
  • On/Off or Variable Speed fan hub – a 2 speed fan hub never fully disengages
  • Dead Axle Tandem – Can improve fuel economy by 3% but sacrifices traction and resale
  • Predictive Cruise Control (PCC) – +2% Fuel Economy

Once you have finally dialed in the perfect spec and you’re ready to put it up against the competition, make sure that there is a fair comparison. There are a lot of factors outside of the spec that can influence fuel economy.

Driver:  One of the biggest influences on fuel economy is behind the wheel. Even the most fuel-efficient spec is going to disappoint if the driver is inexperienced or unmotivated to drive for fuel economy. Make sure that the driver is taken into account when comparing fuel economy between trucks.

Route: Another consideration is the route. If the compared trucks aren’t driving the same route, there is potential for the comparison to be skewed. Routes with more hilly or mountainous terrain typically will result in a lower fuel economy number.

Break-In: One of the more overlooked elements of a fuel economy comparison is the break-in period. Once the engine and tires are fully broken in, fuel economy can improve up to 9%! A typical break-in period is 100,000 miles.