The challenges of measuring fuel economy

With Run on Less in the works, I’ve spent a fair amount of time working with our team on MPG lately.  We’ve looked at questions such as:

  • What’s the proper way to measure it
  • How closely should you track it
  • What is a good MPG
  • Who does and doesn’t want anyone to know their actual MPG

Measuring MPG and knowing what is a good number are both difficult. There are specific test procedures including SAE J1321, coast down tests and more that can be used. These are useful for controlled tests, but they don’t work that well when measuring how a fleet is actually doing in real world operation.

The problem starts with the measuring process. The easiest measurement is taken from the CAN bus using telematics. It’s a well know fact that CAN bus fuel consumption is an estimate based on the engine maker’s own algorithm. It’s usually optimistic.

Some fleets calculate MPG using the formula of fuel purchased divided by miles. This method clearly ties MPG to a fleet’s operating cost, but it only gives long-run averages.

Knowing when to measure also presents a challenge. If you simply measure over the course of one day, you really don’t learn much. Measuring over a week is better, but things get complicated if the truck has crossed from sea level over mountains. This makes it difficult to compare one truck to the other especially if they are operating over different terrain.

External factors have a big influence on MPG:

  • Payload
  • Increase or decrease in elevation
  • The driver (who can influence fuel consumption by 30% or more)
  • Temperature

On top of that there is poor information sharing. Truck OEMs can’t promise a specific MPG, and technology suppliers often put performance into terms that a fleet can’t directly test. And fleets often don’t want to reveal their actual MPG

All of this leads to fleets having difficulty measuring MPG and comparing results. With all the confusing information in the market, it is difficult to define what is good MPG.

Despite all these hurdles, we will be measuring MPG during the upcoming Run on Less challenge. We’ve spent a lot of time working out a way to do it and are currently testing to see if our method works.

We’ll share details with you as soon as we can. In the meantime, we encourage OEMs, technology providers and fleets to continue to collect MPG data, analyze it and share the results so everyone benefits.

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