Fuel tank capacity - real or estimate?

The owner’s manual for my 2002 Acura RSX says the tank is 13.2 gallons. However, I just put in 14.1 gallons at the gas station. Are the manuals usually an estimated fuel tank capacity? How do I determine the real capacity?

In order to put 14.1 gallons into the tank, did you force more gas into the tank after the station’s pump clicked off for the first time?

If the answer to that question is yes, then that tells us that you overfilled the tank. Unfortunately, the result of frequent overfilling is expensive damage to the car’s Evaporative Emissions System.

Don’t worry about the “real capacity” of the tank.
Just get into the habit of refilling the tank as soon as it gets to the 1/4 mark on your gas gauge and remember to stop filling the tank as soon as the station’s pump clicks off.

The way they determine fuel tank capacity for the comic book in the glove box, they send the first pre-production car off the line out to a gas pump and fill it…When the nozzle clicks off, the driver reports back that it held 13.2 gallons…Some makes and models might get a better reading than others. It’s not something that gets a lot of attention…

Thank you,

I just want to be sure that when the pump says that I paid for 14.1 gallons, my tank actually received 14.1 gallons.

Caddy man, did you make this up ? ;=). I thought engineers or 8th graders might as well pour water from the tank into gallon jugs.

You are forcing extra into the tank.
Air space is designed in. you are filling it by virtue of gravity. After a little exchange of trapped air the extra in the filler neck eventually gets into the tank.

Back in the day there were two fuel capcities for the 87/90 Ford Escort. 13 or 11 gallons.
There was just one part number for the gas tank !
The capacity was restricted by the vent tube in the fuel pump and would simple leave more air space when the trapped air would force the incomming fuel up the neck to click off the pump.
Why they chose to do this I really have no clue, and it made selling fuel pumps a real pain in the …tank.

Regarding “forcing” - with gas prices as high as they are, I do not feel compelled to force as much gas into the tank as possible. Once the nozzle “clicks”. I’m done!

On the other hand, I do want to have better confidence in my tank’s capacity and the honesty of the station.

The last time I filled the tank (when the needle got to E and the light was on for about 20 miles of extra driving), I put in less than 13 gallons (upon reaching “click”). This time, under similar circumstances, yet with the light on for less than 5 miles, I put in over 14 gallons.

Contrary to appearances, it is not my habit to wait for the pretty orange light. I agree that staying above 1/4 tank is the wise approach.

Might also be a difference in fueling equipment.
The type and brand of nozzles, and the varying sensitivity of the shut offs .

The true experiment as to station/pump accuracy would take a detailed study on your part using a calibrated container and multiple tries at different pumps and different stations.

For the sake of your car, fuel at the same pump for several tests and see what the readings are then.

Try going to more than one station and see if you get different results. You will probably have to run down to when the low fuel light comes on. If there is a difference, then maybe you should stop using the gas station with the high gallon addition.

JT - I’m thinking the same thing and will go back to using my usual station instead. If I still get a 14 gallon fill-up (despite trying to avoid the orange light). I’ll report back.

There is often a slope in the pavement at filling stations and a few degrees facing upward on one fill up and a few degrees facing downward on the next or vice versa can make a significant difference.

I really see little reason to be concerned about the capacity. If you are concerned about being cheated, every state has a weights and measures department. Their job is to go around and check scales, gas pumps, etc. for accuracy. If not they shut them down. This is usually done at least one a year so it would be highly unusual for a pump to be off by much.

While I do not spend free time reading owners manuals, if I remember correctly the owners manuals I have read have all stated “about, approximately, kinda, within a smidge, etc.” It does not to be exact, just don’t run out of gas.

If you want to check the accuracy of service station gas pumps, you need a carefully calibrated one or five gallon container…Your cars gas tank is just not made to check the accuracy of fuel dispensers…

“I just want to be sure that when the pump says that I paid for 14.1 gallons, my tank actually received 14.1 gallons.” In most states they check the pumps from time to time. I see them around here about once a year. They have a certified container and they fill it. Yes, they do catch a few every year.

Note:  You really don't want to top off your tank.  Doing so can damage the tank on many cars.  I suspect that your car has a note on it or in the owner's manual telling you not to top off the tank.  

But as it has been pointed out, the only real issue is did you get the fuel you paid for.  That makes no difference if you bought one gallon or 20 gallons.

Rod Knox is right. Fuel tanks are seldom if ever regular prisms. And, even if they were, this inclination factor along with having a filler pipe that can hold gas in varying amounts sometimes makes it impossible for the manufacturer to predict Exactly how much gas you can get into a tank on fill up. If you have a lawn mower, or outboard tank whether it be internal or external you can verify this just by tilting the tank a few degrees, making a difference in how much you can fill it. I’m sure there are some iregular shapes out there where the tank can hold more, not less when on an incline. I would not be surprised also if atmospheric pressure also had influence on the fuel pump shut off.
An interesting discussion but the manufacturer is NOT to blame for in concise fuel tank capacity. It’s gravity’s prerogative.

It’s not difficult at all for the mechanical engineers to calculate volume on any and all parts they design. On Pro-E it’s a few button clicks away. They know down to a ten-thousandth the dimensions of the parts as designed. And they know the capability of the operations used to manufacture the parts so they specify the tolerance. The ability to gauge the volume on any shape, regardless of complexity, has been available for 20 years or more on even rudimentary CAD software.

The variables contributing to the inaccuracies have been well covered above.

The capacity in the manual is supposedly what the tank is designed to hold. There are several factors that may allow you to fill it more:

-You may be squeezing gas into the filler pipe or designed in air space.

-The pump may not be calibrated correctly, despite the auditor’s seal. The temperature of the fuel may affect its density as well and fool the pump.

-The manual may have a typo or you may be reading the capacity for a different model–eg. the 4-cylinder version vs. the 6-cylinder equipped.

-One way manufacturers trim weight from a vehicle to meet standards is to shave fuel capacity off by shrinking the fuel tank. Since it would cost to have the manufacturer get a whole new redesigned fuel tank from a supplier, it may be the capacity is listed as lower to meet weight requirements, but the fuel tank will actually hold a bit more. This is my opinion, but working for an automotive supplier, I wouldn’t be a bit surprised.

Todays electronically controlled fuel pumps can be manipulated to overcharge the customer by as much as 15%. Checking them whit a 1 or 5 gallon container will not work, these are the size of the containers that the authorities use to check the pumps accuracy.

The pumps are sometimes controlled through a little switch under the counter. The counter person can switch to make the pump more accurate when they see an auditor, or anyone filling an external container, others remain accurate up to 5 gallons and then start metering more gas than they deliver. Most people buying gas are buying more than 10 gallons.

I always buy gas at about the same number of miles. I know haw much gas I can put in depending on the season, a little more in the summer and dead of winter, less in spring and fall. I have occasionally come across a station where the fill up is a gallon more than I expected. If it happens twice at the same station, I stop using them. Discount stations are usually the worse for this.

Even at that, I have never come close to putting in the amount listed as the capacity of the tank. About a gallon less is the closest I’ve ever come. I generally fill shortly after the idiot light comes on, within 20 miles.

“The pumps are sometimes controlled through a little switch under the counter. The counter person can switch to make the pump more accurate when they see an auditor, or anyone filling an external container, others remain accurate up to 5 gallons and then start metering more gas than they deliver. Most people buying gas are buying more than 10 gallons.”

For this conspiracy to work, the guy working the gas station register (and getting paid minimum wage for a job likely to get him on the wrong end of a gun) has to be such a “good company man” as to be trusted to keep his mouth shut, no matter what.

It seems more likely that the worker would spill his guts the second he got fired, which does happen from time to time. Or perhaps try to blackmail his now-hated ex-boss.