What is the best way to measure fuel economy?
Dashboard display? Or fuel purchase records?

Fuel purchase records because it measures moving averages in a real-world environment.

And what could be simpler? Number of fuel gallons consumed divided by miles driven.

You can get results immediately by pulling the past six months of fuel purchase receipts and grabbing a calculator.

On all vehicles?

Take a sampling. The best way to do this is to collect information on a group of tractors in your fleet. Make sure the tractors are the same age, make and haul similar loads over the same routes. For example, you will not get the outcome you want if you compare a 2007 Kenworth with a Cummins engine and automatic transmission against a 2004 Peterbilt with a Detroit and 18-speed manual.

TMC recommends reviewing at least six months of fuel economy data. Minimum. One test is an indication, not a trend.

Using the 6 months of fuel purchase receipts, find the gallons consumed and divide by miles driven = miles per gallon.

What about drivers?

A driver's attitude and skill-set can affect fuel economy by as much as 35 percent. Do you have time-conscious or fuel-conscious drivers? Do they possess defensive road manners or passive? While it may be a bit of both, try to include in your profile tractors that are driven by drivers who have similar behind-the-wheel experience and share the same habits.

And then?

Now you know your fuel economy. If one tractor drops 35 percent, you know there's a problem. If you want to test a new tire, you have a real-world figure for comparing performance. If someone tries to sell you the latest and greatest widget, which they say can save you up to 10 percent on fuel, you have the hard data to prove or disprove their claims.

TMC recommends reviewing at least six months of fuel economy data. Minimum. One test is an indication, not a trend.

But this method has its flaws.

Sure. But the flaws are the same for everyone. It's fair. The calculation uses moving averages that even themselves out. 

For example, Bridgestone follows TMC/SAE guidelines when it conducts fuel economy tests. One of the procedures directs fuel to be weighed and measured - Bridgestone doesn't drive to the nearest truck stop for fuel.

It's safe to say that it's unrealistic for your drivers to weigh and measure fuel for both tanks each time they fill up.

That's why we recommend using the rate of consumption - the amount of diesel fuel consumed by your tractors. The variables that change results - such as weather conditions and routes - tend to even themselves out.

TMC reports ECM data is not standardized for: idle time, average speed, percent time in cruise control, percent time in "high gear," time above company speed limit, panic stops, and fuel consumption.

Why not use the dashboard display?

Some fleets use the information presented in the dashboard display to represent their fleet's fuel economy.

However, TMC recently held a panel discussion, and the experts concluded that all ECMs, which feed information to the dashboard displays, are different.

There is no industry standard for ECMs. So the TMC does not recommend this method for measuring fuel economy.

How do most ECMs measure fuel consumption?

Most measure the volume used in each pump stroke. But the calculation doesn't account for temperature, fuel density or energy content.

What does "energy content" mean?

The amount of energy available in a fuel source. Every type of diesel fuel ignites and burns differently. Ultra-low sulphur diesel is different from biodiesel. Winter diesel is different from summer blends.

High temperature fuel has less energy content than low temperature fuel too, so what you buy on a hot day or from an above-ground storage tank may not be as full of energy as cooler fuel.

Another difference is how each manufacturer "counts" the amount of fuel used during idle. TMC reports that idle fuel consumption - as rated by the each manufacturer's ECM - can vary by up to 10 percent.

What if we still use the ECM to calculate fuel economy?

Then it's best to use it as a side-by-side comparison for measuring truck A performance to truck B. It may not be accurate to use ECM data as a fleet-wide measuring tool.

You may also compare the same tractor under different conditions. If you slip-seat drivers, you may see a significant difference between them. Or you may be able to compare different loads, routes, seasons, etc.

Dashboard data has one unexpected benefit. It's a number that's constantly in front of drivers, challenging them to let off the throttle, smooth out their driving and stay within the speed limit.

Any other comparisons?

One member of the TMC panel compared ECM data to manual methods for several models, and he found ECM fuel consumption information ranged from plus five percent to minus seven percent of actual fuel consumption.

This raises the question, how can you rely on a measuring tool that varies that much?

Who was on the TMC panel?

Two engine manufacturers and one private fleet manager. The panel moderator was from a worldwide shipping company.  The panel discussion was published in The Trailblazer, TMC's member magazine, in February 2005. If you're a TMC member, you can access this article online.

What about readers who are not TMC members?

TMC has given Bridgestone permission to e-mail this article to REAL ANSWERS readers. If you wish to receive a PDF of this article, e-mail realanswers@trucktires.com