Join Date: Apr 2018
Location: Northeast Alabama
Car: 2007 2.5 L Obsidian Black Outback XTL
Feedback Score: 0 reviews
Available fuel is based upon a measurement of level in the tank. It is not quite as simple as saying that 12 inches equals 10 gallons so 6 inches equals 5 gallons or 3 inches equals 2.5 gallons. That type of calculation is based upon a cylinder or a cube where there isn't any usual contours to the walls or the floor of the tank. The fuel tank in a Subaru is an irregular shape and as it gets narrower near the bottom there is much less fuel left based upon depth of the tank.
To measure this type of tank you either use a mathematical formula to calculate the volume (like with an sphere or a pyramid) where the shape is regular. A gas tank in a car is irregular, there there is a tapered floor and a lower area (in the world of tanks, this is known as a sump). To determine the value of this type of tank based upon depth you would use something known as a "strapping table" that is based upon actual measurements of remaining fuel based upon depth. Then this strapping table is built in to the fuel calculation code and you allow the code to interpolate values between the lines in the table. It roughly approximates the true value (more accurate when there are more entries in the strapping table).
I can see where someone let a less diligent engineer make some assumptions to the programming (maybe they did not use enough granularity in the table) or they have a zero vs. span problem with the sensor (that happens too). Usually you can get pretty darned close by calculation and generic tables; to be super-accurate someone would need to calibrate the depth sensor to each individual tank (that is NOT going to happen).
I am a geek; I was a process controls engineer for many years and this was the kind of stuff that I did every day.
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