Fuel Economy Methods, Part 1

How do you calculate fuel economy? I read so many different claims about fuel economy, and I keep coming back to wondering how people arrive at their data. For instance, on the diesel forums it is not uncommon to see someone claim 24 MPG in a diesel pickup. And yet I have personally driven a Duramax LLY with a heavy load up a grade, and watching the digital mileage indicator, I have seen instantaneous readings as low as 3 MPG. That’s a huge swing.

We dedicate a lot of effort to correctly measuring fuel economy during our engineering evaluations. Fuel economy is simple to calculate – distance traveled divided by the fuel consumed equals miles per gallon.

How do I accurately measure the distance traveled? If I rely on my odometer, I must assume that it is completely accurate, but what if I have installed larger than stock tires on my vehicle? Unless I have done something to compensate for this in the odometer reading, it is going to be inaccurate. You could even argue that excessively worn tires would produce an inaccurate odometer reading. These days, I like to use my handheld GPS unit to give me an accurate distance traveled and then compare it to the odometer reading.

How do I know how much fuel is consumed? If I fill the tank up to the point that the fuel nozzle clicks off, is it really full? I’ve tested vehicles that have taken as much as two additional gallons after the nozzle has shut off, just by playing with the dispenser and filling the neck to the top. It’s a pain, but at least I have a starting point that I can return to when I refill the tank. If I am doing a before-and-after comparison test, I should also make sure to use the same pump and park the vehicle in the same position for consistency.

I’ve only scratched the surface…more to come.

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