MPG reading error?

Asked by Apr 08, 2017 at 01:00 PM about the 2016 Toyota Prius

Question type: General

I bought the Toyota Prius 3 in November and mostly love it. The GPS really
sucks though. My question is about the MPG readings. I reset the trip A
meter every time I fill up and last time it says that for the 753.7 miles I
traveled on that tank, I got 77.4 MPG. When I filled up at that point it only
took 10.45 gallons of gas. By calculation that comes to just over 72 MPG.
By rough estimate, I noticed that it was about 5 mpg less than it says every
time I filled up.So is the meter wrong? Or am I being cheated at the pump?  

2 Answers

74,105

I would believe the GPS over the regular odometer. There is no guarantee that the fuel meter in your car is absolutely accurate either.

35

Great job with your hyper-miling. The Prius trip meter, odometer, speedometer, and fuel efficiency calculations use wheel rotations not GPS readings. Your Prius believes it used 753.7/77.4 = 9.74 gallons of gas. Using different cars and different gas stations, I've found that cars often systematically over or underestimate fuel consumption more often than gas stations do. A gas station could make illegal profits by mis-measuring, but their measurements are tested by the state department of weights and measures in most states. Cars don't measure the fuel flow so precisely. Usually they don't measure it directly at all, only infer it from air intake flow or manifold absolute pressure and oxygen levels in the exhaust. If this discrepancy is unusual for your Prius and gas station, the pump probably filled the tank with about 0.7 gallons more than the last time you filled up. Hopefully the tank didn't quite fill the previous time rather than overfilling at this gas station. Please don't top up or let others top up after the pump stops filling. Doing so risks filling your charcoal canister with gas, fouling it, leading to evaporative emission pollution and costly repairs.

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