Why does the bottom half of a tank of gas seem to go faster than the top half?

Question

I have noticed it takes longer for my gas gauge to go from Full to 1/2 than it does from 1/2 to E. Is this just my imagination? Thanks for any input.

Answer 1

here is an article from www.cartalk.com
It was from a person that asked the same thing back in 1999. If you have never been to this website they are the same guys that in the sunday paper answer all kinds of auto questions.


Dear Tom and Ray:

While I know I should be worrying about the tension in Kosovo, I am instead trying to figure out the answer to a dumb question. Why do I get many more miles out of the first half of my tank of gas than out of the second half? For example, my car gets about 200 miles out of the first half-tank of gas. You would think, then, that I would get 400 miles from a full tank. But I don't. I get only another 100 miles or so as the needle goes from half-full to empty. This has been true of other cars I've owned, too. Why is this? -- Frank

Ray: You're absolutely right, Frank. It IS true of every car I've ever owned, too.

Tom: I can't say whether or not my '63 Dodge Dart ever did this. I never filled it up all the way because I didn't want to invest more money in the car than it's worth.

Ray: Here's what's going on: Even after your gauge reads "Full," the tank itself is not necessarily completely filled up. You can always cram in an extra gallon or two. Sometimes more. So you've got gas at the top end of the tank that the gauge doesn't even see.

Tom: So right away, you're starting with a couple of "bonus" gallons at the top. Then, you get "cheated" out of a couple of gallons at the bottom end of the tank, because even when the gauge reads "Empty," there's still more gas in there. And the amount varies tremendously from car to car.

Ray: Here's how it may work using hypothetical numbers. Let's say the actual capacity of your gas tank is 20 gallons. The gauge may only show you what's between the 18 gallon mark (Full) and the 2 gallon mark (Empty). The gauge would read "half-full" when you had 9 gallons left.

Tom: So after you've "filled it up," you have to burn off 2 gallons of fuel before the needle even starts to move down from the "full" mark. That means you actually DO go farther on the first half of your tank-full. And by the time you've gotten down to the halfway point (from 20 gallons down to nine), you've had the benefit of 11 gallons of gas.

Ray: Whereas, in the second half, you only get 7 gallons before the gauge reads empty (from nine down to two). And that's only if you run it all the way down to empty, which most people don't.

Tom: Why do most manufacturers make their gauges like this? It's not that they can't make them more accurate. It's just that they don't want you to overfill the tank or run out of gas.

Ray: Overfilling the tank can ruin the charcoal canister that traps fuel vapors. It's part of the emissions system and is therefore the manufacturer's responsibility for the first 100,000 miles.

Tom: And they don't want you to run out of gas for two reasons. One is that, as we all know, they are great humanitarians and don't want you to have to walk home on a cold, dark, rainy night. But more importantly, the car's fuel pump lives in the gas tank and is cooled by the gasoline. And they'd feel just terrible if you ran out of gas and burned out the fuel pump -- especially while the car was covered by THEIR warranty.



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Answer 2

The rate of fuel consumption is neither higher or lower regardless of the amount of fuel in the tank. The fuel gauge sending unit can only go so high (full on the gauge).

For example: If your car uses $20 of fuel from the half full point on the gauge to empty, then you used that fuel up and put $40 of fuel into the tank you would probably be on full. But if you "filled" the tank to the very top... then your car may take $45-50 in gas.

Your car takes longer to come off the "full" mark and then registers more accurately how much fuel is in the tank.

Another example would be that the fuel gauge sensor is only in the tank and when you fill your car right up, you are filling the gas tank neck as well, something that the fuel gauge cannot read.

Hope this makes sense and helps.

Answer 3

that's not purely an city legend. The flow contained in the motor vehicle is designed for the flexibility of the TANK ... no longer the filler tube ensuing in the tank. once you fill your tank, you commonly provide up even as the pump stops (i.e. the gas reaches the nozzle). mostly, that's the bubbles that set off the air rigidity change contained in the pump. Then, many human beings 'perfect-off' with yet another squeeze or 2 after the bubbles die down. the base line is that you've actual one to one and nil.5 extra gallons of gas sitting contained in the filler pipe and overflow chamber (once you've one) than the tank became measured with. also, even as the tank get's on the breaking point of empty, the gas sloshes round and the sensor famous itself out of the fluid and sends a demonstration to the vehicle. the vehicle (mostly computing device pushed at the moment) surely takes and person-friendly. Yeppers, that's the actuality.

Answer 4

Its a combination of a few of these answers. The gas level in your car is measured by a float. the float is on an arm. Think of your arm, if you stuck it straight out and started at your hip, I know its reversed, but for an example work with me...lol. Now start at your hip. Your hand being the float lift your hand straight out in front of you. Thats your half way point...every thing below your hand is the top half, your legs, belly, feet...quite a bit, now whats left in the remaining half? Yup much less than half. Just your head neck and shoulders. Obviously your fuel thank isn'st that uneven, its much closer, but the principle behind it is. Good question....(I had to take apart a gas tank to find this) LOL Nah not really, i was a mechanic and had to do it anyway. LOL

Answer 5

Besides the psych factor which is: "I've got plenty of gas, look, it's over half full to I have less than half a tank and it is getting closer to running out as we drive", fuel tanks are fitted into the vehicles and they are not perfectly shaped boxes or fuel containers. The measured inch deep volume of fuel at the top is different from the inch deep volume in the middle and different again from the inch deep volume at the bottom. The gage is designed to compensate for it somewhat but only generally.

Hope that helps

Answer 6

A very good question. I noticed that too. My explanation is that the gas meter is designed that way, meaning the top half actually refers to more gas than the bottom half.

Answer 7

It is because the bottom of your gas tank is curved so is actually holding less gas. It happens with almost every ones cars.

Answer 8

Because the sending unit is float controlled. It is easier for the float to work down hill with out the restriction of gas. so it comes off faster.

Answer 9

its the gas gauge your car isnt burning the gas any faster

Answer 10

I have no idea, but that is a great question!!! I have often wondered this myself! I will have to check back later to see if anyone knows the answer!