At what level do you fill your car's fuel tank?

[randomness]

[Poll #1714922]

Comments

[sarakate.livejournal.com]

My mom always refills hers when it gets to 1/2 on the gauge, because she *firmly* believes that it gets better mileage in the top half of the tank. No amount of explaining how the floater in the tank works can dislodge this belief; it's clearly an article of faith and not a reasoned position.

This is made *much* funnier when you know that my mom has degrees in math and petroleum engineering. :)

[ceo]

That's not an unreasonable conclusion; on my last car and my current one, the needle doesn't budge off the peg for the first 100 miles or so, then drops much more quickly once it's down past ½. Is that due to how the floater works, and if so, how does it work?

[awfief.livejournal.com]

It is due to how the floater works, and I can't get into it now, but CarTalk did a recent (in the past 2-3 months) spiel on it, and they have a podcast of their episodes....so you can find it :D

[sarakate.livejournal.com]

Yes, that's due to the floater. It floats so that part of it is above the level of the fuel, so as you're filling the tank, it will touch the top of the tank before the tank is actually 100% full. Then, as you use the gas, the gauge registers full until you get below that point again -- basically, F on your gauge means "the floater is touching the top," rather than actually "the tank is full". When you get into the lower "half" of the tank, it falls more quickly because the bottom of the tank isn't flat -- it slopes toward the outflow pipe, so that the gas flows from the tank into the pipe even if your car is headed uphill and the tank can drain completely, and therefore the same unit drop in volume causes a greater unit drop in floater height, which is what the gauge is actually measuring.

[sarakate.livejournal.com]

And I don't expect everyone to know that, or even necessarily to get it when first explained, but my mom taught *me* geometry and made a living for 30 years as a reservoir engineer.