In my 06 Prius I can squeeze in just over 10 gallons once the light comes on or about 60 lbs. With all the electronics in the car (and I've added ScanGauge for even more) I reset the MPG at the beginning of every tank and have noted repeatedly that for the last 1/4 tank or so I am able to get the average to go up. I have attributed this to the 50 or so fewer lbs the car is hauling around at that point.
Yes the Prius exibits the same goofy behavior on gauge movement. I know that at one point I went over 300 miles before the gauge moved off FULL. Yah I love good mileage but that's just silly.
In my wife's 2002 the thing goes from 3 bars (of 10) to one bar in about 20 miles. 450 miles on the first 7 bars, 20 on the last 3!
Due to this silliness, I pay no attention to the fuel gauge except if I plan on going a fair distance to judge if I can make without having to stop.
It doesn't need to be this way either. We had a boat for years that held 12 gallons. When full the needle exactly covered the full mark. When it was straight up at half tank exactly 6 gallons would fill it. When the needle perfectly covered the "E" the motor quit, it was quite remarkably accurate. (Yes we always carried spare fuel on the water!)
I'm guessing on a boat, packaging is less of an issue than on a car. I think most fuel gauges still use a float, although I'm not sure about that, so they're actually measuring fuel height in the tank, not fuel. If you've ever seen a gas tank (here's a pretty extreme example: http://www.ineospolyolefins.com/Imag...uel%20Tank.jpg), you can imagine that fuel level really does not track fuel volume very closely.
I'm no expert, so you can take my opinion for what its worth, but here's my take on this. Your cars fuel system is designed to run cycles from full to empty, meaning that its okay to wait until the light comes on to tell you that you need fuel, if you filled it to full to start. Manufacturers tend to agree because the manuals for all the vehicles I have ever owned simply say to not drive vehicle below E as to not run out of gas. My point is that it is okay to fill your tank up, then drive till the light comes on. Just fill up as soon as possible after that.
Partial fill-ups have two downsides, and one upside. The upside is that you carry less weight, effectively increasing MPG's. This I feel is negligible, since I drive a Titan with a 28 gal. tank, and have not noticed a difference in MPG at any level of fill up. The first downside has already been discussed. Constantly driving your vehicle with less than a 1/4 tank of gas will cause the sediment in your tank to clog your pump, lines, filter, injectors, or any combination of these components.
The second downside is what I think may lead to MPG variance based on when, and how much you fill your tank. A common misconception about fuel systems is that its a one-way feed. Gas is pumped from your tank, through the filter, to the injectors, and into your cylinders for ignition. However, fuel systems are not one-way, they are loops. Fuel is pumped from your tank, to the filter, to the injectors, where only a small portion of what is in the lines is fed into the cylinders. The rest returns back on to the tank where it awaits to be pumped back to through the loop. Fuel pumps are electric motors, and like any motor, electric or not, they generate heat. Fuel pumps are designed to cool, or at least not overheat, by using the fuel that it is pumping to absorb some of the heat. So your fuel heats up as it travels into the engine compartment and close to the engine, returns to the tank, and then heats a little bit more as it passes back through the fuel pump. This creates a never-ending cycle of a little heat, a lot of heat, then sitting in the tank to cool. If you constantly have low fuel, it won't have time in the tank to cool, therefore it will continue to increase in temperature, and at some point, on longer drives, actually cause the fuel pump to increase in temperature!
Now, if you run your vehicle until the light comes on, as long as roughly 75% of your driving time is done with more than a 1/4 of a tank, your fuel pump was designed to get hot a some point. But if 75% of the time you have less than a 1/4 of a tank, your fuel pump with be hot all the time. My friend has a 98 prelude, and refuses to ever put more than $10 at a time, usually $5. He is on his 3rd fuel pump in 3 years and he still doesn't believe me.
Anyway, sorry for standing on my fuel pump soapbox, but I know I was very surprised when I learned how all that works. As far as when and how much fuel to put in your tank. If you always go from 1/2 a tank to full, your fuel has more time to cools as it cycles through the system. Cooler fuel is more dense, and therefore most newer cars have the technology to sense the density of fuel, not having to deliver as much volume to meet the engines needs. If you fill from empty to 1/2 a tank, your fuel will not have as much time to cool, your fuel will be less dense, meaning the engine must deliver more volume in order to run. I'm no physicist, so its just my opinion, but it makes sense to me.
PS: I've hear a myth that you get more fuel for your money if you fill up in the morning or at night because the its cooler outside, making the fuel more dense. Gas pumps measure volume, so the denser the fuel is, the more gas you really get per cubic inch. Also just a myth, I've never paid attention to this, but I might do some experiments with that next summer.
..g2gsr93: i think the morning/night fuel-up theory was proven inconsequential (check snopes.com, i think they have something on that there)..
..the fuel-loop was interesting, as it implies (just to further my original theory) that a fuller tank can absorb heat more readily than a less-full one..and since gasoline tends to evaporate (at any temp) or go less dense (at hotter conditions), it makes sense that running towards Empty means you've got less fuel to cool the gas returned to your tank..
..granted this is a not-an-expert opinion squared (since i'm less expert than anybody & using it as a consideration point), but it's something to think on..