Do car owner manuals, generally speaking, give the practical (usable) gas tank capacity, or the absolute capacity.
On the rare occasions when I have trouble finding a station and run the tank lower than I would like before filling, I wonder if I can just subtract the amount I use to fill from the stated capacity and find out how much I really had left to run on, or if that is a mysterious quantity and I’d need to actually run the car out of gas, then fill it up to know it’s true capacity.
The manuals I have checked do not hint about the nature of the gas tank capacity spec.
You’d need to run out (not recommended - it’s hard on electric fuel pumps). The stated capacity is a nominal figure, and probably even figures on a certain amount of air space inside the top of the tank. It’s not a precise volume measurement.
Now, gas pumps are rather precise, and generally are checked and certified annually. I learned this, and the imprecise nature of gas tank specs, when the amount I pumped into a car exceeded the spec.
Unless you’re using the same gas pump every time, each pump arrangement and angle of the asphalt at the pump will slightly alter the maximum capacity of the vehicle.
“Furthermore, the rated capacity may include only the usable portion of the fuel tank or it may include both the usable and unusable portions. The unusable volume is the portion of the fuel tank’s liquid capacity that lies at the bottom of the tank out of reach of the fuel pump.”
I’m willing to put up with some variation, but it would be nice if the manufacturers had to say which way they are measuring.
The Oregon link is right in saying it’s not really standard either way. I’ve heard I think with equal frequency complaints that people ran out of gas before they were supposed to or that they were able to fill up the tank more than the listed capacity and therefore the gas pump must be crooked.
The trouble is that the usable capacity may be hard to quantify. With a fuel injected car, you’re effectively out of gas when the gas starts sloshing away from the pickup. So you can get to a point where you can squeeze out a few more steady-speed highway miles, but if you’re driving around town with lots of starts and stops it’ll cut out too much. Not that doing either of these things are very good for your fuel pump.