Donating Fuel upon Full Tank? How true is this?

**Arshxd** · July 28th, 2013 11:45 PM

Tips on Pumping Gas - Urban Legends

According to this 1% per 15 degrees. But because gas is underground it doesn't matter.

**jut703** · July 29th, 2013 12:03 AM

I work for an oil company and I find this claim quite misleading, at the very least.

First off, when having your car filled up (i.e. full tank in automatic), what happens is that the pump dispenses fuel into the tank continuously. When the tank is full, the fuel in your tank touches the sensing tube (located at the end of the fuel pump nozzle) which closes a mechanical valve in the nozzle, cutting off fuel supply. Think of it as you filling a bucket with a garden hose - when the bucket is full and the water in it begins to touch the tip of the garden hose, the water supply is automatically cut. Because of this feature, it's very unlikely that you will be "donating" fuel back to the underground tank.

If you're concerned about the fuel left in the hose, this actually isn't counted in your meter so you're really not losing anything that you pay for. If it helps, these fuel dispensers are calibrated several times in a month manually, with very low tolerances for error. So far I've yet to encounter a pump that severely under-dispenses.

Now, onto the topic of vapor reducing the amount of fuel in your tank - to some extent, it's true. When filling up your tank at high speed, it's indeed possible to have bubbles form in your tank (as mentioned, similar to pouring coke too quickly). However, also like a bottle of coke, the air that forms the bubbles doesn't come from the nozzle itself, but from the surrounding air, so you're not "cheated" since all that's being counted is the liquid fuel from the pump. That said, it's important to let the fuel settle down first in your tank when filling up to the brim (sagad, not just automatic) because you get to maximize your tank that way. However, in general I don't recommend filling a tank to its full limit because of the volatile nature of gasoline, which needs a certain ullage (allowance of air) inside the tank.

As for the temperature affecting the amount of fuel filled up - again, this is true but I want to qualify some things. Yes, fuel expands when heated so it's less dense at noon than during a cool night, but keep in mind that fuel is stored underground and are enclosed in very thick steel (or fiberglass) tanks which means that changes in the fuel's temperature aren't as drastic as changes in ambient temp (think of the water in a thermos). I haven't done an empirical test on the actual difference of a full tank at noon and at night, but I'm willing to bet that it's not that huge. I say this because the expansion of fuel from 15º (standard measuring temp) to ambient temperature (25-30º) is very minimal (~1%).

Filling up "uno" and choosing the time of day to fill up can indeed make a difference, but it's so minimal that you're better off adjusting other aspects of your driving if you want to be more frugal. Simply turning your engine off while filling up saves you a few mL of fuel that you'd have wasted idling.

**nivraarvin** · July 29th, 2013 12:38 AM

Originally Posted by dr. d

does that translate to 500 meters extra/less travel per 600 km of travel.. ? not much, but it is there..

Let's just say, you saved 2 pesos.

And yeah, filling up at "uno" doesn't really make of much difference if you know how flow meters work.

**jcRN** · July 29th, 2013 10:14 AM

in the article that I posted, "petrol" refers to gasoline or to the diesel as well?

**ghosthunter** · July 29th, 2013 10:23 AM

**Chikselog** · July 29th, 2013 05:08 PM

Saw this post in FB. Funny how a lot of people quickly believe what they read on the internet.

**oj88** · July 29th, 2013 06:31 PM

In most developed nations, they have pumps that capture the fumes as your fuel tank is being filled up. The system is called fuel vapor recovery. The purpose is not to cheat the customer, rather, this prevents the harmful fumes from venting into the atmosphere. Unburned gasoline fumes are dangerous and is a pollutant.

As you know, when your tank is being filled, the air inside (fumes) will be displaced and the only way out is through the filler hole. The vapor recovery system captures these fumes and takes it back to the underground tank.

In HK: EPD - VOC Control for Petrol Filling Station

**Ry_Tower** · July 29th, 2013 06:37 PM

Originally Posted by ghosthunter

Agree, eh sa mga new cars na lang mas makikita ito dahil sa real-time fuel computation. I always fill my ride full pero yung unang click lang at hindi ko na sinasagad. Yun lang gawain ko at sofar tama naman computation ko sa FC.