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When is a Mile Not Actually a Mile (in MPG)?

Updated: Aug 12, 2021

The tracking of fuel economy for a truck is not always a simple calculation of miles traveled vs. gallons used, or MPG. We also need to know how much work our engine is putting into traveling that mile. The work is influenced by many factors, such as the load we’re pulling, wind resistance, air temperature and humidity, accels and decels, road surface and also, very importantly, the terrain on the route we’re following. So, without going into the depth of everything to be considered, let’s look at the ups and downs of the route to see the impacts of terrain.

Here’s an example of a long-haul truck crossing various routes, involving traveling over both mountains and plains.

Route Load Stops Miles MPG Redding, CA to Hagerstown, MD 76K Lbs. Little America, WY 872 4.7

Colfax, IA 941 5.56

Hagerstown, MD 1002 5.54

It’s clear that the terrains had a major effect on the MPGs. We need more detail to see just how much that influence was. At SPI.Systems Corporation, we use a tracker system which clarifies what the engine is feeling at different elevations. Here are some examples:

The green terrain plots tell the full story of every transition in elevation that the engine must respond to. In step with the terrain challenges, lots of variables in the engine are changing. They include net torque, RPM, fuel commands, EGR valve opening, NOx values, DEF dosing, turbo speeds, and intake and exhaust pressures. Terrain changes will result in an impact on MPG. We can’t do much about these transitions, but we can adjust our MPG calculations to more realistic numbers by being aware of not only the terrain changes but also the duration of each major shift, because that is the main driver of our eventual MPG. So, now MPG becomes a rate of fuel usage over a specific duration of terrain value AND distance.

Here’s how SPI measures and reports MPGs that have been adjusted for terrain:

These factors are now loaded into an algorithm that allows us to track every increment of MPG without simply relying on the fuel stop value at the pump.

Why do we care?

When we compile each set of increments, we gain a better indication of the true function of the engine over the course of the entire route. In one example, the MPG varied over the entire route from 4.7 to 5.56. With our algorithm, we are in a position to see why that value changed as we look at the engine parameters that changed at the same time. So, we accomplish a much more detailed view of the various influences upon engine performance and response to transitions.

This engine was equipped with the SPIER System of improved fuel economy and emissions. Its MPG without SPIER ranged from 4.7-5.56, while its performance with SPIER was 5.2-7.98. Every incremental effect was tracked in both cases.

For more information on MPG improvements across all conditions, please visit

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