Are gas pump auto-shutoff mechanisms consistent?

[Pajamarama]

I'm trying to keep very detailed statistics on both my cars' fuel economy. When I fill up, I stick the pump nozzle in the filler hole and let it go until it shuts off automatically. As I understand it, the nozzle will shut off when the vapor recovery pressure reaches a certain amount.

Is this pressure consistent? Can I reasonably assume that if I go to pump A and fill up my car's tank, then later fill up at pump B, it will shut off when the tank is at the same levels?

 

[Coprolite]

If you want to be the most consistent, use the same flow rate setting and same pump.

 

[Artem]

^x2

 

[FZ1]

x3.

 

[RiceCake]

Even at the same pump, it depends on the car, but for the most part, the consistency will very hugely. You need a huge sample size to dial out the variables, or add fuel in metered amounts.

If a bubble forms in the filler neck it can be enough to shut the pump off early in my car, if that doesn't happen it can fill the tank completely before stopping, a difference that is sometimes as much as four litres (a gallon) of fuel.

So don't count on it being scientific at all what you're attempting.

 

[dparm]

Those things are horribly inconsistent. Every vehicle is a little different too; my mom always swore her I30t had a faulty filler neck, as it would almost never auto-stop.

 

[cjcride]

Bet the temperature will also affect the shutoff and MPG.

 

[Virtuoso]

Originally Posted By: Coprolite
If you want to be the most consistent, use the same flow rate setting and same pump.



x4 This is about as consistent as you'll be able to get.

 

[mikefxu]

I had one at a fairly new gas station that didn't shutoff. I would put it on auto and check my fluids. Guy ran over and cut it off for me, he reacted quicker than I could figure out why he was yelling at me. Now I stay by the pump.

 

[Benzadmiral]

When I can, I use the same pump each time. I put in the nozzle, press the regular button, then place the little trigger lock at the second step. Then I watch the readout. When it gets to a point about 2 gallons less than I've figured it will be*, I release the trigger lock and hold the trigger down myself until the pump shuts off.

(* rough calculation using (dash readout of gallons used) * 1.1, since the readout is always about 10% low.)

 

[Nick R]

Originally Posted By: Benzadmiral
When I can, I use the same pump each time. I put in the nozzle, press the regular button, then place the little trigger lock at the second step. Then I watch the readout. When it gets to a point about 2 gallons less than I've figured it will be*, I release the trigger lock and hold the trigger down myself until the pump shuts off.

(* rough calculation using (dash readout of gallons used) * 1.1, since the readout is always about 10% low.)



You lucky people that can have trigger locks. They are illegal in NYS :
I generally use the same pump every time I fill up, and I usually top off to the nearest 25c increment to satisfy my OCD.

 

[badtlc]

From my limited research at 2 stations, it depends on how you define "consistent." If you round to the nearest gallon, yeah, they are. If you get much below that, for me, they do not appear consistent. It appears they can vary by a + or - 0.2 gallons, even on the same pump.

 

[DBMaster]

Around here, in the 80's, I used to notice that they seemed to remove the clip/locks from the pump handles. They started leaving them on with the advent of vapor recovery nozzles. I guess the idea was to try to discourage "topping off" that leads to spills.

My 89 Accord had a shallow angle filler neck that used to splash out gas upon shutoff frequently - and, I never used anything past the second detent on the clip. If I paid attention I could hear the sound of the tank getting close to full and I could stop it manually. The Mazda does not have this issue.

 

[gofast182]

After you do it enough times you should have a very good idea of how much the tank will take. If you combine that with using the same pump whenever possible you should be very accurate in your numbers. Beyond that I wouldn't worry too much. How much could the auto-shutoff trigger really vary, a couple tenths of a gallon, max? For example, my TSX has an 18.5 gallon tank which usually gives me a range of about 500 miles in mixed driving. With that capacity and range it shouldn't be a worry. When I got the car, a few manual checks told me the on-board range and MPG calculations are dead-on.

 

[badtlc]

if looking for precise measurements, i'd first verify the pump actually pumps 1.0 gallons when it says 1.0 gallons. once you have an accurate pump, you can start verifying other measurements/calcs. Pumps are so inconsistent with regards to shutoff points, single tank hand calcs mean very little even using the same pump.

Upon learning all of this, when I wanted to start keeping precise MPG records of my car, I realized the onboard MPG calculator was the easiest way to remove almost all the variables that can cause hand calcs to be less accurate.

I ended up doing like gofast182 above me to test the accuracy of my onboard MPG calculator except longer. I found an accurately measuring pump and used it exclusively for 3 tanks to eliminate noise in the measurements while not resetting my AVG MPG readout over this same time period. This ended being about 1200 miles of data. Using the total gallons from those three fillups and the total mileage over those 3 tanks, it turn out my onboard MPG calculator was correct to the 0.1 MPG. With that established, it makes MPG comparisons much easier since I can ignore what pumps I use from that point on.

 

[SwampSurvivor]

We don't have trigger locks any more. There are a few gas stations where I can only get 7/8 of a tank in without it constantly clicking off ... then there are some other tanks where I can get the needle to read over full.

I generally get gas, at the same station on lunch.

 

[SteveSRT8]

In our area some have trigger locks, some do not. You can still stuff a gas cap in there and the auto shut off will work (if it works at all!).

All my current cars and fleet trucks seem to have no issues with hungrily drinking thousands and thousands of dollars worth of delicious fuel every month!

 

[Durango]

Originally Posted By: Pajamarama
I'm trying to keep very detailed statistics on both my cars' fuel economy. When I fill up, I stick the pump nozzle in the filler hole and let it go until it shuts off automatically. As I understand it, the nozzle will shut off when the vapor recovery pressure reaches a certain amount.

Is this pressure consistent? Can I reasonably assume that if I go to pump A and fill up my car's tank, then later fill up at pump B, it will shut off when the tank is at the same levels?


Pajamarama,

I wouldn't always trust the mechanism. The gas station is suppose to keep them in order but not always! I've had precious gas spill out of my door because the auto shut off failed!!! Here in California failure to keep the auto shut offs in working order is big time illegal.

When it comes to my gas and my money I hand squeeze my gas by hand and keep my eye on it until it's full. My personal motto....never trust a machine too much.

Durango

 

[jrustles]

Originally Posted By: Nick R
Originally Posted By: Benzadmiral
When I can, I use the same pump each time. I put in the nozzle, press the regular button, then place the little trigger lock at the second step. Then I watch the readout. When it gets to a point about 2 gallons less than I've figured it will be*, I release the trigger lock and hold the trigger down myself until the pump shuts off.

(* rough calculation using (dash readout of gallons used) * 1.1, since the readout is always about 10% low.)



You lucky people that can have trigger locks. They are illegal in NYS :
I generally use the same pump every time I fill up, and I usually top off to the nearest 25c increment to satisfy my OCD.


is it also illegal to use the gas cap to lock the pump on?

 

[Doog]

Originally Posted By: Pajamarama

Is this pressure consistent? Can I reasonably assume that if I go to pump A and fill up my car's tank, then later fill up at pump B, it will shut off when the tank is at the same levels?


No.