How do you measure fuel economy

Ask just about anyone what kind of fuel economy they get, and the answer will be “xx miles per gallon.”  The federal government mandates corporate average fuel economy ratings for vehicles of 10,000 pounds or less, and your senior management probably wants to know your fleet’s miles per gallon (MPG) numbers. While traditional MPG ratings probably make some sense for cars – and to a lesser extent, over-the-road trucks – they have very little to do with the typical work truck application. I know it will be a hard sell, but I suggest vocational truck fleet managers need to develop a new standard for how fuel economy is measured.
Think about it – if your trucks are designed to perform a specific job, the significant factor is how much fuel is consumed per unit of work accomplished. This form of measurement would also eliminate the issue of energy density differentials among various fuels such as diesel, gasoline, natural gas and propane. The critical comparison factor would become cost per unit of energy instead of gallon equivalents – which is confusing to just about everyone. State of the art electronics could easily be developed to measure energy consumption with the most likely energy unit being the Joule (metric) or British Thermal Unit (BTU). For comparison purposes, one BTU is equivalent to approximately 1,055 Joules.

Measuring work
The scientific definition of work looks at three factors: weight, distance and time. So if nothing moves, no work has been done. For example, the definition of one horsepower is the lifting of a 33,000 pound weight one foot in one minute (33,000 ft.-pounds per minute). In most cases this would not be a valid or realistic way of measuring the work done with trucks, so I can foresee the development of different measurement criteria for different industries. This does not really impact the overall concept as long as each industry agrees on common measurement criteria.

To give you a better idea of where I am going with this concept, here are some potential work measurement benchmarks:

  • Bulk transport – Ton mile or gallon mile (of product) per Kilojoule
  • Package delivery – Average number of packages delivered per Kilojoule
  • Power-export applications (PTO-driven equipment) – Kilowatts of usable PTO output per Kilo-joule (power export conversion efficiency measurement)
  • Route delivery – Number of customer stops per Kilojoule

Putting it to work
Looking at fuel consumption in terms of energy per unit of work accomplished makes it significantly easier to evaluate the impact of various sustainable technologies and/or alternative vehicle and equipment designs on overall operating costs. For example, let’s consider the addition of a number of weight and energy-saving technologies on a delivery truck (high road mileage with minimal stationary power demands) starting with wide base, single, low-rolling resistance tires. For this example, I will assume that the truck grosses out before it cubes out. If we know the current energy per unit of work measurement (EUW) and take the tire manufacturer’s low value for improved fuel economy of 2%, we can immediately assume a 2% reduction in EUW costs (ignoring the associated weight reduction), which is no different than assuming a 2% reduction in fuel costs. However, wide base single tires can provide as much as 250 pounds of more weight reduction per wheel end. If we look at a tandem rear axle truck, then this means a usable payload increase of 1,000 pounds. If the truck originally had a net payload of 30,000 pounds, the 1,000 pounds of increased payload translates into a 31,000 pound payload – a 3.3% increase. There will be no increase in the gross weight of the unit, so baseline fuel economy is not impacted – but the EUW goes down by approximately 5.3%. It is then a very simple calculation to determine the dollars of energy savings associated with the use of the wide base single tires.

If the fleet in question is also considering the addition of aerodynamics to their trucks, we can again quantify the baseline reduction in EUW but now must factor in the potential reduction in payload due to the weight of the technology. The baseline impact of the reduced payload (increased EUW) must now be offset by the reduction in aero drag. Again, by looking at total EUW, the evaluation becomes much simpler to make and at the same time, more accurate.

Conversely, if the truck in question is primarily utilized to support/transport work equipment to the job site and performs the majority of its work in a stationary mode, rolling resistance and weight reduction would have almost no impact to the selected EUW measurement. There may be a slight increase in the MPG rating, but since the vehicle probably drives a minimal number of miles, and most of the energy produced by the engine is utilized for power export (PTO operations), we should be more interested in technologies that will directly reduce the EUW factor for the specific application.   

A change of mindset
Measuring work truck fuel usage by EUWs will require a new mindset, but I see it as a means of focusing our energy conservation efforts on the specific areas/technologies that will have the greatest impact on work truck groups based on how they are actually used. This concept may never gain acceptance, but I offer it to you as a way of getting you to think about what is really important in your specific applications when evaluating energy conservation and efficiency technologies.

If you would like to discuss this or any other fleet issue with the NTEA, please call 800-441-6832