Well?
My Lil’ Geo can get to about 300-350 km when the gas needle is at half, but only about 200 -250 from the second half. Why?
I also drive a Geo. It’s the gas gauge that isn’t true. When the needle is on Full, I can still get between half a gallon and a gallon of gas into the tank. So by the time the gauge says 1/2, you’re really down to about 3 gallons of the 8 gallon tank left.
Maybe you’re not running it all the way out, go back and test it again.
The universal law of gas gauges is that they are not linear, even radial, whatever.You know what I mean.
Sunbear:
I’m afraid I don’t know what you mean.
Well, here why this seems so. Neglecting the fact that the guage may not be a direct linkage to the float in the tank, nor geared at a 1-1 ratio. Typically the float (or non mechanical level indicator) reaches only the top of the tank at its location. The trick is that the tank is not perfectly symetrical. Second the gas usually fills the tube running from the gas cap to the top of the tank, an unmetered location. Finally you likely don’t run your car very close to truely empty. Just as the needle can go well beyond “F” it also usaully can go well beyond “E” something that wise drivers don’t do.
My car illustrates this well. I can get about 380 miles/tank that is leaving about a gallon in the tank (figured by subtracting the measure from the pump by the listed volume of the tank). The trip meter reads about 80-100 miles before it even reaches the “F”. 125 miles or so between “F” and 1/2 tank, and about the same from the second 1/2. Then after pushing another 40-50 miles on “E” I fill up at the nearest 24 hour pump. Although trusting the trip meter, while more accurate than the gauge, can be dangerous when you get some cheap watered down gas in the sticks somewhere.
Deja 'vu all over again.
voodoo days are here again
This was covered in a book once by David Feldman, I believe.
Anyway, I think its the other way around, the second half lasts a lot longer than the first half.
The Car Talk people explain modern gas gauges as a way to keep you from running out of gas. On some car models, the fuel pump is in the gas tank. As this is a bit to believe off hand, I hope the following at least suggests I didn’t make it up.
I’m wagering the second half of this thread is going to take a hell of a lot longer than the first.
Miss Gretchen:An inch of gas is not the same as Joule of psi in the fuselage, when the odometer is on Imperial Gallons. OR: As the others explained, half on the gauge is not a half a tank.
I had a feeling that’s what you meant, sunbear. But the doorbell rang and I just hit “Submit reply” without elaborating. If it sounded snotty, well, actually, hell, this is the SDMB, I should be a little snotty now 'n again! 
I recently got an 84 Chevy Caprice, and marked down the odometer reading when the tank was full (i.e., immediately upon filling it with gas), and when the needle got down to F, to 3/4, to 1/2, to 1/4, and just barely E. I did the about 3 or 4 times, and concluded that the tank held 20 gallons (not counting whatever was still in the tank when it got to E), and that I drove the same distance in each interval, give or take only 5-10%. Thus, each quarter-tank on the gauge represented about 4 gallons, and another 4 gallons were in the “above-F” area.
So from a full tank to midway was 12 gallons, and only 8 gallons from midpoint until I started panicking about running dry. I hope this helps.
During college, I drove a '68 Mercury. When I filled up the tank, the gas needle would slowly move towards E as is the custom. BUT once it got to E, it would then move back towards F! It would do this until it got to the 1/4-mark and then move back towards E again! I always had to remember if the needle was on its first or secind trip towards E?! Needless to say, I ran out a few times before I figured out what was happening.
“Quoth the Raven, ‘Nevermore.’”
E A Poe