Estimating “empty”

In addition to recording gas mileage data, I also recorded the number of miles driven after the tank was supposedly empty (“Miles after ‘E’”). Here’s what I did. Once the “E” (empty) indicator activated, I reset one of the trip odometers. Then I drove on until some convenient time to refill the tank, while keeping an eye on total miles driven and keeping in mind the Prius’s estimate of gas mileage (“Displayed Mileage, mpg”). I just wanted a ballpark figure for about how far I could drive on “E.” (I went too far one day about three months after I got the car. Details here.)

My data suggest that the tank has about 3 gallons of gas after the gas gauge shows “E.” That means I can easily drive the car another 100 miles. That’s the theory. When I wrote this on June 30, 2007, the data showed that I refilled the tank way before 100 miles went by. Eighty seems to be my outside comfortable limit. From a chart of the miles driven after “E” on the fuel gauge, I apparently refill the tank well before that.

Some background about the Prius. The gas tank supposedly holds 11.9 gals. However, the fuel tank is a soft “bladder” made of some synthetic material contained within a hard container. It is not a metal tank, as with a conventional gas tank. So, unlike a conventional gas tank, the volume of gas is not set. According to a car salesperson, “ya never really know if you’ve filled the gas tank to capacity.” P.S. I’m one of those who “tops” off the tank—mostly to the nearest dime in terms of price. With this “bladder” gas tank, I find I’m adding another, oh, half gallon or so of gas after the gas pump automatically shuts off the first time.

The Prius bladder gas tank shrinks and stiffens in cooler temperatures. Conversely, gas expands in the heat. A posting on the Internet says that Toyota claims the difference in bladder tank capacity on a warm day versus a cold day can be as much as 1.3 gallons: “The fuel tank capacity is decreased at low ambient temperature (decreased by about 5 L [1.3 gal., 1.1 Imp. gal.] at -10C [14F].)” All of this and more can affect the mileage displayed by the Prius’s on-board computer—and what you calculate as your actual mileage. (See displayed versus actual mileage.)

The best way to determine the capacity of the Prius gas tank is to average the fill-ups over a long period of time (i.e., over many, many fill-ups).

It looks like I’m filling the tank every 410 miles (average). A chart of miles between fill-ups shows that I can comfortably drive about 450 miles on a tank of gas during the summer, less during the winter. The operative word here is “comfortably.” Under ideal conditions, the maximum number of miles from a tank of gas seems to be about 540 to 580 miles during the summer, and between 480 and 520 miles during the winter. Safer to assume max 450 miles during the summer.

Note: Your mileage may vary!

Go to gas mileage chart.