How do you analyze the fuel economy for the trucks in your fleet? There are a variety of different ways to measure fuel economy and not all of them yield the same results. The approach to measuring fuel economy should coincide with the type of analysis you are conducting.

One approach is to use data from the engine’s electronic control module (ECM). If you go this route, you need to be aware that different OEMs use different algorithms to compute fuel economy. This could be a problem if your goal is to compare the fuel economy of two different trucks — and the algorithm on a Freightliner, for example, is slightly different than that of a Mack.

Using the ECM to measure fuel economy requires the appropriate software for the type of engine being evaluated. The software needs to be installed on your laptop and the laptop must be physically plugged into the ECM on the truck to download the data.

When measuring fuel economy, it’s important to compare apples-to-apples. You will want to download data from the same time period within the same domicile location and application. You won’t get meaningful results if you compare fuel economy of one truck in the winter with another in the summer; there are too many variables with the impact of externalities such as weather, road conditions and fuel blend. If you have 100 trucks in your fleet, you have to touch each one at the same time if you want a complete understanding of your fuel economy numbers.

A benefit of downloading ECM data is that you can obtain a lot of other good information, like idle time and drive time in a certain gear. Obtaining a history of how the truck was driven and how the engine performed cross-tabulated with the trucks MPG is a great analysis on an individual truck.

You can also use a third-party onboard computer (OBC) to measure fuel economy. These devices communicate the data to a website. There is usually an in-cab display that allows drivers to see their performance, too. Fuel economy data (and limited truck performance data) is communicated several times a day using cellular technology to a website. This is one of the most common ways fleets measure fuel economy, but the downside is each third party OBC has its own algorithms for computing fuel economy.

But now you have two measurements — ECM and OBC — that don’t have standard guidelines of how fuel economy is measured.

The good news isyYou can quickly get the data you need using this method.

The bad news is you need to use multiple algorithms. This is not a good method for comparing one make truck to another make truck, and depending on the reliability of the cellular signal, some of the data downloads may not get reported.

While this may not be an optimum way to compare one make of truck to another, it can be quite useful if you are trying to measure the individual performance of a truck over time and comparing the performance of same make trucks over time.

Personally, my favorite way of measuring fuel economy involves a simple mathematical formula: miles driven divided by gallons of fuel pumped. If your sample is big enough, you can eliminate outliers that may exist because of human error, such as misreading an odometer or fuel pump. And you can always cross-reference it with either of the other methods. It is a great way to compare one truck to another but it does depend on excellent recordkeeping.

There is no one right answer when it comes to measuring fuel economy. But since fuel can represent up to 70% of the cost of operating a truck — excluding driver costs — it’s important for a fleet to get its arms around the fuel economy numbers.

Fuel economy is a large factor in total life cycle costs and by regularly tracking it with sound methodologies, fleets can see when a tractor starts to experience fuel economy degradation. This data is very useful when determining when the time is right to retire an asset, and a good benchmark to help the fleet determine which tractor to buy next when adding to its fleet or replacing an existing unit.

Understanding the fuel economy of the tractors in your fleet is important, whether diesel fuel is $2 or $4 a gallon.

Decide what it is you are trying to do — compare one tractor to another or, track the fuel economy of an asset over time or, do a comprehensive performance analysis on a single unit — then select any one of these methods to get the answers you need.