My boyfriend and I are masters of useless information. We’re always spouting off random facts to each other. Unfortunately he has a “fact” in his repitoire that makes absolutely no sense, and he refuses to let it go.
He says that a car’s gas tank is bigger on the top half than on the bottom half. He says this is true because if you fill up your gas tank and measure from full to the halfway mark, you can drive more miles than you can from the halfway mark to empty.
Now, to me, this makes absolutely no sense. Saying one half of something is bigger than the other half totally defies the definition of “half”. Two halves of something are equal. If they weren’t equal, they wouldn’t be half. If it appears that one can drive more on the first half of a full tank of gas, then it’s probably just because you can’t tell when you’ve exactly reached the halfway point, and also because you never drive a car until it completely runs out of gas. There’s always some left when you fill back up. Therefore it would seem that the first “half” would be bigger. There are also way too many variables involved. What if you drove predominately on the highway during the first half, and then puttered around town during the second half? The gas mileage would be different, therefore making it seem like you could drive more miles on the first half.
This is seriously bugging me. I either need proof that a) this “fact” has absolutely no logic behind it, or b) he actually is right. Either way, I need proof, because we’re both skeptical of just about everything and need actually legitimate information before we believe something.
The first part is true, tanks are shaped (vaguely) like funnels, for obvious reasons. I’d expect the fuel gauge to be able to accurately measure volume rather than simply the height of the fuel, but you never know with cheaper car makes.
I think he means that the miles you get out of the car on the first half of the gas gauge are greater than the second half. That is true for my SUV by a noticeable margin and I have seen it on other cars as well.
It seems like a design flaw. Of course, that has nothing to do with “half the gas tank”. It is only the gauge and it could be designed to read any way the engineers choose.
Another problem is that gas guages are not always reliable. I had one vehicle where the guage would stay above the “F” mark until the tank was about 3/4 full. If you ever got down below the 1/4 mark you needed to fill it up or you would run out.
I used to build gas tanks. They come in all shapes and sizes, and there’s no way at all determine what half is a half when you’re talking about a half. Take a look at a current Mustang tank and you’ll see what I mean. There is no top half to speak of.
The first thing that I thought of when reading the OP was, “well, mileage won’t be constant over the period of emptying the tank… carrying a full tank will weigh the car down more.”
Of course, I don’t really know how much this factor affects the results.
So, basically the consensus is that the gas level is measured by a floater, and since gas tanks are usually funnel or cone-shaped, halfway full may not necessarily mean that the gas is half gone, just that the level of gasoline in the tank has reached a point halfway between the top of the tank and the bottom of the tank.
Gas tanks are not usually funnel or cone shaped. Generally, they’re somewhat box shaped. But that’s not particularly relevant, because the issue here is that automotive fuel gauges are not designed to be precision instruments. This is really the crux of the matter, so let me repeat it:
AUTOMOTIVE FUEL GAUGES ARE NOT DESIGNED TO BE PRECISION INSTRUMENTS.
Official testing procedures for fuel gauge systems specify the sending unit having a certain resistance when its float is hanging all the way down, and certain different resistance when the float is held all the way up. The gauge should read empty and full, respectively. Sometimes there’s a fine adjustment to make sure the empty reading is right on the mark. There’s no mention whatsoever of any in-between position.
The “1/4,” “1/2,” etc. markings are simply VERY ROUGH guides to the general level of fuel in the tank. The float position, the resistance of its potentiometer at that position, and the corresponding gauge needle position are not meant to, not designed to, and do not accurately reflect the proportion of fuel available in the tank, and are not linear with respect to it.
Your boyfriend’s assumption that the reading corresponds accurately with the geometry of the tank in some way is wrong. Just flat wrong.
We have covered this a few times here before. The difference is tiny, like really tiny. Gas will only be a few percent of the vehicles weight and the engine mainly is concerned only with getting that weight moving. On an open road at constant speed, it wouldn’t really affect gas mileage at all.
Do some (higher-end) cars have more sophisticated gauges? It doesn’t seem it would be that hard to have a gauge that would say, “At your current speed, you can travel another 231.7 miles before you run out of gas.” Maybe something sensitive to the pressure or weight of gas in the tank? I mean, gas must have pretty consistent weight, right?
Oh yeah - if your boyfriend would like to assert what he actually observed, that more miles can be driven between the full and 1/2 readings on the gauge than between the 1/2 and empty readings, and the obvious correlation that more gas is consumed going from full to 1/2 than is left in the tank at that point, I’d agree wholeheartedly. If he were talking about readings on the gauge, he’d be right. When he makes the erroneous assumption that said reading is the same as volume in the tank, he’s wrong.
My parents 1991 Plymouth Voyager would tell you how many miles you could go before you needed to fill up. It was fun to watch when you got down to about 50 miles, because it would go from 50 to 51 to 53 as you went down a hill, only to decrease to 52 then 50 then 47 as you went up the next hill. Stopping at a stoplight would cause it to drop even more precipitously. On a couple of occassions, we even tested what would happen if you got down to zero. (Answer: I don’t think we ever saw a zero, or ran completely out of gas, but it would stay for amazingly long times at 1 mile.) I don’t believe that this display gave tenths of miles.
The other thing this story illustrates is the truth of Gary T’s statement. The gas gauge, and this readout, were not designed to be precision instruments.
Aside from the terrible inaccuracy of gas gauges (which haven’t been changed in 50 years) there’s two more things that haven’t been stomped on yet.
The float on the in-tank arm will get heavier over time, and will change the way your gauge reads a half-full tank. After the float cycles up and down a few hundred times, the electric resistance will change from wear on the wire. A half-tank when the car was new is not the same as half a tank now.
Gas tanks are designed to have some air space at the top to allow for expansion. There are baffles in there to keep you from filling the whole tank with gasoline. Also, the sensitivity of the nozzle’s shutoff will vary. So, the “fullness” of your tank may vary as much as three gallons from fill to fill, especially if you gently sqeeze in more fuel after the shutoff. You can nudge the back of the car to slosh fuel into some of the baffled headspace to get even more into a “full” tank.
Raz, your boyfriend’s theory is based on incorrect assumptions, so it’s invalid, from full to empty. We’ve all done bonehead reasoning like that, so don’t rub it in too hard. Your turn will come.
As usual Gary T nailed most of it.
A few points I would like to bring up. Gas gauges are just slightly more accurate than sticking a stick down into the gas tank, but this does not mean they are crude instruments. A modern fuel gauge is buffered, and computerized, but even so it is an estimate.
Buffering is where when the fuel level changes very quickly (more quickly then it could from consumption along) the needle does not reflect the change. Back in the 60’s I owned some British sports cars that had unboffered fuel gauges. If I had about a 1/2 tank of gas I could use the fuel gauge as a G meter. around a right hand corner the harder I cornered the higher it would read, around a left the lower it would read. Nowadays people won’t accept that so the guages will not respond to a sudden change for several minutes. If you leave your key on and fill it up, you will notice that it takes a couple of minutes before the guage reads full. This is the buffering in action.
As my co-worker Balthisar says they come in all shapes and sizes. We even have some that have a big hump in the middle of the bottom for the drive shaft to pass through. This type of tank requires two fuel pumps and two fuel senders and a compute to determine what the correct fuel level is. In addition some tanks the float won’t reach all the way to the top, and/or all the way to the bottom of the tank. The computer then has to estimate just how far above or below the float the level really is.
Now with all that said I really have to laugh at the OP’s boyfriend. Numerous times I have heard people brag about how they can drive 50 or a hundred miles before their car drops off full, but it runs out as soon as it hits empty. Then someone else mentions how they can drive for 100 past empty, but the needle comes off full right away. I would guess that over the years, I have heard about an equal number of each of these.
One last point. If you keep toping off the tank past where the nozzle does the auto shut off, you will be putting fuel in the expansion space above normal fuel level. This of course will extend the time it takes you to get to 1/2, but does nothing for how long it takes to go from 1/2 to empty. That does not mean the system was desinged that way.
