MPG readings on cars dash or scan gauges?

[Letxen3]

i have a question about how the digital MPG readings work on recent cars or on scan gauges.

for example is it a calculated number or does it really match up is one is to do the real way on calculating it (filling up and taking the miles driven divided by the gallons put in on next trip)?

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[Paul]

My car has both the instantaneous MPG reading and the running average since the last refueling stop. The running average is within a few percent of the traditional method of using the trip odometer and the volume of fuel added.

The MPG reading is calculated/estimated from data from the engine management system. The instantaneous value is not the most informative, especially when you’re accelerating up a hill or decelerating from the highway. One could also make an estimation from the tachometer.

The running average is somewhat inaccurate just after you fill the tank up. Of course, the accuracy improves as it averages more driving data.

[Bryan Carter]

Modern cars calculate MPG in a very direct, yet round about way. Your car’s ECU knows for certain how many milliseconds each injector is open. Assuming that fuel rail pressure is within factory specs, it can calculate how much fuel is being consumed at any given instant. From there it’s just a matter of taking a gander at the vehicle speed sensor… and presto! You have your instantaneous MPG reading.

Now when it comes to your average MPG, each automaker applies their own proprietary “smoothing algorithms” to the raw data to keep the reading from fluctuating so much that it would be distracting to the driver. They apply these algorithms to most electronic gauges because bouncing needles tend to make buyers bring cars back to the dealership. They don’t want that. Even before the days of electronic wizardry, mechanical gauges contained various dampening strategies to keep the needles smooth.

[Author]

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