are fuel economy gauges doing math wrong?

March 26, 2013 at 3:27 pm | | stupid technology

A few years ago, there was a piece in Science magazine about how we should all be using GPM instead of MPG. Here’s a link to my post about it back then. The main point is that the relevant unit for fuel use should be fuel divided by distance, and than MPG is inverse, which is harder wrap our puny brains around.

Back then, I looked into how fleet fuel economy averages were calculated, because I was worried that one single crazy-high MPG model could artificially skew the average high without actually making the fleet more fuel efficient. It turns out that the US requires automakers to calculate their fleet average fuel economy the correct way: convert each model’s economy to GPM, find the mean, then take the inverse again. Phew.

(not my car)

But now I wonder if the computer programmers at the automakers know how to calculate an average. My computer fuel gauge is always inflated compared to what I calculate from the fuel pump amount and odometer reading. Every time. And I’m not alone. So, either the gas stations are messing with their pump readings, or the average MPG on my dash is miscalculated. I wonder if the computer just takes a continuous average of the measured MPG values, which would definitely result in an inflated number at the end of a gas tank. (That’s because 10 miles of driving at 35 MPG after 10 miles at 25 MPG does not average to 20 miles at 30 MPG. It’s actually 29 MPG.*) If the computer just averages the MPG numbers, it will be 2% inflated from the actual value. That inflation would get worse if you drive down a hill for a mile and go 50 MPG, then back up that hill and drop down to 10 MPG: instead of an average of 30 MPG, the true average is only 17 MPG!

Of course, if the computer just takes the total miles driven and divides by the total gallons of fuel used (since the reset), than the average would be calculated correctly. But is that what’s happening?

Does anyone know anyone who works at an automaker who can check how they do the math?


* Here’s my math. Driving 10 miles at 25 MPG uses 0.4 gal of fuel. Driving 10 miles at 35 MPG uses around 0.29 gal of fuel. That’s 20 miles driven and 0.69 gal fuel used, or 29 MPG. Not 30 MPG. For the hill example, down the hill uses 0.02 gal for the mile, and up uses 0.1 gal. That’s 2 miles and 0.12 gal, or 16.7 MPG.

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