Gas evaporation

Hi, I got a 2002 civic LX 4 door. I really enjoy the gas mileage on this CAR but recently I realized something that I would like to find out what is happening in the gas tank. While I was watching how fast the needle for the gas tank drops I saw that it takes 125 miles for needle to reach 3/4 mark (on the gas gauge) and then it only takes about 70-75 miles for needle to reach 2/4 mark. I start to wonder if there is a “gas evaporation escape problem” in my fuel system.

No, you don’t have a problem with gas evaporation.

The problem is that your gas gauge–just like all of the gas gauges–is not a laboratory-grade measuring device. Because it is not an extremely precise device, the needle does not drop in a linear fashion as the level in the gas tank drops.

On most cars that I have ever owned, it has taken a much longer time for the needle to show a decline for the first 1/4 of a tank or so, and then it sometimes seemed to drop like a rock by the time that it indicated 1/2 tank.

What you have observed can be considered to be, “normal”.

@matteous I believe you’re getting yourself worked up about nothing

Here’s how to calculate fuel economy

Go to the filling station
Fill up until it clicks off
Reset the trip odometer
The next time you fill up, fill only until it clicks off
Divide the number of miles driven by the amount of fuel to top off
Don’t worry too much about how fast the needle moves
If your numbers are close to the EPA numbers, you’re doing fine

FWIW . . . many cars have fuel gauges that behave as yours does.
Doesn’t mean anything’s wrong.

Yes, there’s often a lot of gas above the ‘F’ mark. Old car of mine had about 1/5 above ‘F’, 1/5 for each ‘quarter tank’ from there.

It’s also difficult to tell – unless you run the tank dry and mark it with a Sharpie pen – where on the guage the gas tank is empty.

But I look at it like this. Accurate or not, as long as the guage is consistent, it’s way better than nothing. Many early era cars didn’t have gas guages. I guess you just kept a spare can of gas with you in the car back then. Even the VW Beetle didn’t have a gas guage. I think with the Beetle, when it ran out of gas, that didn’t mean you were out, it was a warning, so you flipped a lever and you had another gallon or so. For around town driving, even today, in a pinch that method would work.

A lot of motorcycles have the reserve “tank” system.
In many cases it’s not a separate tank, it’s just a pocket in the bottom of the tank with its own drain.

not all gas tanks are symmetrical. Most are oddly shaped and the bottom half of the tank might be smaller than the top half. As the float in the tank drops, when it gets to the smaller portion of the tank, it’ll drop faster than the larger top half.

My Crown Vic will go 150 miles from “full” to 3/4 full…But when it gets down to 1/4 full, you are lucky to go 75 more miles before you start walking…

It’s the shape of the tank and how the tank sender is set up inside the gas tank…

@circuitsmith- They found an even cheaper way- the petcock is fed by two tubes, one longer than the other. Rotate the valve to select which tube feeds the line. Single assembly to install. I’ve had bike tanks where the design dictated separate halves and they have no choice but to have a line from both sides. Then they outfit one with the longer tube…

Nice to have reserve on system with no gauge but also a bummer when you reach down to find it’s already in reserve position!! \$#@%&\$!

Keep in mind that in general manufacturers are designing cars for their usual driver. Most drivers never compute their mileage manually, if at all. That gas gauge is there only to tell the driver that they are close to running out of fuel, and they are not worrying about how accurate the gauge is.

``For most drivers, it is enough to know that it is close to full, maybe about half full or close to empty.``

VDC is right, all my Hondas read the same way as yours. The sending unit for the gauge is a float attached to a wire that pivots from a single point. The path it follows is almost a 1/4 circle, start at the top, a full tank, and it follows the arc as the gas goes down. So at first the sending unit is almost going horizontally, but the lower the tank gets the more vertical it goes. This may be a bit simplistic, but that is basically what’s happening. the lower the tank, the faster the gauge drops.

The gauge linear, but the tank is not. If the gas tank had a flat bottom and straight sides, then the gauge would be accurate over the entire range, but in most cars the bottom slopes, so what shows as the last 1/4 tank is actually way less volume than the first 1/4. In my '82 Rabbit the “1/4” marks were offset to compensate for this, but alas, most design committees opt for symmetry over accuracy.

Gas gages are as reliable as politicians promises. I have never owned a car with a reasonably linear gas gage that could be relied on to accurately see what whas in the tank. I do have warning lights on both cars that tell me I have about 2.5 gallons left. My wife always plays it safe and fills up when the gage is 2/3 down.

No one has stated the obvious, on most cars, when you fill the tank, the needle goes past the full mark. You may be driving anywhere from 25 to 40 miles before the needle drops to the F and then another 70-75 miles before it goes from F to 3/4.

In summary, the gas gage is not designed as a precision measuring device, only as a reference to give you an idea when it’s time to add fuel. Each gage is uniqe, and none is intended to tell you volumetrically how much fuel you have…or have used, but rather as a rough idea when to plan to start looking for a gas station.