# Are gas pumps really accurate to 1/1000 of a gallon?

**URL:** <https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258>\
**Category:** Factual Questions\
**Created:** [June 1, 2016, 10:32pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258 "2016-06-01T22:32:02Z")\
**Posts on this page:** 12\
**Page:** 1

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**Author:** ![Shoeless](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/shoeless/32/7099_2.png) [@Shoeless](https://boards.straightdope.com/u/Shoeless)\
**Post date:** [June 1, 2016, 10:32pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/1 "2016-06-01T22:32:02Z")

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A while back, I was filling up my car at a very slow gas pump where I could actually see the hundredths of a gallon distinctly changing on the display, instead of the blur that it normally is. I thought to myself, at least the 3rd decimal place is zipping along faster than I can see. Then I got to thinking, why the heck does a gas pump need to go out to three decimal places anyway?

I know that gas pumps are regularly calibrated and checked for accuracy, but are they really accurate to a thousandth of a gallon? If I’m doing the math correctly, that works out to 0.128 ounces. That’s less than a teaspoon!

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**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\
**Post date:** [June 1, 2016, 10:43pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/2 "2016-06-01T22:43:03Z")

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Perhaps they though it would give people a sense of reassurance that it’s a precision device and that they are getting exactly the amount of fuel that they are paying for?

There’s a related trope in film & t.v. where super-smart sciency types are portrayed as giving a ridiculously inappropriate number of significant figures. “Captian, I calculate a 37.462% probability that you will survive your fight with the alien species that we have just encountered and know pretty much nothing about.”

In reality, the number of significant figures stated tends to be inversely correlated with the person’s intelligence and level of scientific understanding.

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**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\
**Post date:** [June 1, 2016, 10:48pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/3 "2016-06-01T22:48:29Z")

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[Maybe.](https://www.quora.com/How-accurate-are-the-pumps-in-gas-stations)

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**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\
**Post date:** [June 1, 2016, 11:12pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/4 "2016-06-01T23:12:24Z")

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To quote the significant point in **beowulff’s** link:

\*\*"So the pump is precise to 0.001 gallon, because it can measure and record volumes in increments that small.

But the pump is not [necessarily] accurate to one thousandth of a gallon:

Your particular gas pump may be spot-on, or it might be a little off. But the maximum allowable error is only 0.3%. You can usually trust your local Department of Weights and Measures to keep gas pumps in line with reality."\*\*

I added the “necessarily” to improve the accuracy (;)) of the statement.

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**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\
**Post date:** [June 1, 2016, 11:18pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/5 "2016-06-01T23:18:20Z")

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> [@Gary\_T](#):
>
> I added the “necessarily” to improve the accuracy (;)) of the statement.

It never occurred to me before, but if you’re debating an issue and somebody keeps repeating the same dumb argument over and over again, I guess the right response is “Precisely!”

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**Author:** ![Shoeless](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/shoeless/32/7099_2.png) [@Shoeless](https://boards.straightdope.com/u/Shoeless)\
**Post date:** [June 2, 2016, 12:10am UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/6 "2016-06-02T00:10:30Z")

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So I should have asked “Are gas pumps really _precise_ to 1/1000 of a gallon?”

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**Author:** ![PastTense](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pasttense/32/14550_2.png) [@PastTense](https://boards.straightdope.com/u/PastTense)\
**Post date:** [June 2, 2016, 12:17am UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/7 "2016-06-02T00:17:52Z")

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I think it is also true that gasoline expands slightly with the heat.

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**Author:** ![mmmiiikkkeee](https://avatars.discourse-cdn.com/v4/letter/m/edb3f5/32.png) [@mmmiiikkkeee](https://boards.straightdope.com/u/mmmiiikkkeee)\
**Post date:** [June 2, 2016, 3:25pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/8 "2016-06-02T15:25:54Z")

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So yes the pumps really are precise down to the milliliter (using liters as is done in Canada)… though that’s not necessarily the exact amount that enters you gas tank. I always notice a bit of gas that will dribble out the end of the delivery tube depending on how you jiggle or angle it. It only takes about 20 typical “drops” to make up a volume of 1ml, and more than that will dribble out of you let it. Still though, it’s a tiny amount and probably one of the better measured products you’re likely to buy day to day.

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**Author:** ![Fridgemagnet](https://avatars.discourse-cdn.com/v4/letter/f/22d042/32.png) [@Fridgemagnet](https://boards.straightdope.com/u/Fridgemagnet)\
**Post date:** [June 2, 2016, 3:40pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/9 "2016-06-02T15:40:58Z")

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> [@PastTense](#):
>
> I think it is also true that gasoline expands slightly with the heat.

It does, which is why race car fuel allowances are usually specified by mass rather than volume. If you fill up your tank first thing in the morning when the tanks are cool then you’ll get your fuel ever-so-slightly cheaper.

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**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\
**Post date:** [June 2, 2016, 4:15pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/10 "2016-06-02T16:15:25Z")

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> [@Fridgemagnet](#):
>
> It does, which is why race car fuel allowances are usually specified by mass rather than volume. If you fill up your tank first thing in the morning when the tanks are cool then you’ll get your fuel ever-so-slightly cheaper.

Surely a myth - An underground 20,000 litre tank will not change volume overnight.

One think I do know for a facts is - never fill right up, drive a short distance and park for a few hours in the hot sun.

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**Author:** ![mmmiiikkkeee](https://avatars.discourse-cdn.com/v4/letter/m/edb3f5/32.png) [@mmmiiikkkeee](https://boards.straightdope.com/u/mmmiiikkkeee)\
**Post date:** [June 2, 2016, 6:11pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/11 "2016-06-02T18:11:16Z")

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> [@bob\_2](#):
>
> Surely a myth - An underground 20,000 litre tank will not change volume overnight.

Not necessarily a myth… it depends on a bunch of factors and what you mean by “ever so slightly”. [This](http://datagenetics.com/blog/april32015/index.html) pages discusses the details, and brings up some of the relevant factors such as how fast that underground tank is turned over, what temp the gas was that went into it, and temperature compensating devices used in some areas. I don’t know what kind of tanks race car fuel is held in… whether they have their own small above ground tanks like those on farms, or whether they have massive below ground tanks or actually just go to the regular gas station near the race track.

And again, do you define an “ever so slight” amount as $5 per fill up or 0.5 cents?

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**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\
**Post date:** [June 2, 2016, 6:17pm UTC](https://boards.straightdope.com/t/are-gas-pumps-really-accurate-to-1-1000-of-a-gallon/756258/12 "2016-06-02T18:17:36Z")

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> [@Shoeless](#):
>
> A while back, I was filling up my car at a very slow gas pump where I could actually see the hundredths of a gallon distinctly changing on the display, instead of the blur that it normally is. I thought to myself, at least the 3rd decimal place is zipping along faster than I can see. Then I got to thinking, why the heck does a gas pump need to go out to three decimal places anyway?
> 
> I know that gas pumps are regularly calibrated and checked for accuracy, but are they really accurate to a thousandth of a gallon? If I’m doing the math correctly, that works out to 0.128 ounces. That’s less than a teaspoon!

Good answer [here:](https://www.quora.com/How-accurate-are-the-pumps-in-gas-stations)

> [@Ryan Carlyle @ Quora](#):
>
> Sure, it can measure a thousandth of a gallon. Sort of.
> 
> One one\<script id=“gpt-impl-0.09065125351247644” src=“[http://partner.googleadservices.com/gpt/pubads\_impl\_88.js](http://partner.googleadservices.com/gpt/pubads_impl_88.js)”\>\</script\>-thousandth of a gallon is a little less than a teaspoon (~4mL). That’s a very small volume, but it’s plausible to measure with a gas pump’s flowmeter. Gasoline pumps usually use piston-type positive-displacement flowmeters. They’re the gold standard for metering fluids for sale transfers.  
> The precision of the meter is determined by the manufacturing tolerance on the seals, the size of each piston’s fluid displacement, and the angular resolution of the rotary encoder. A standard modern gasoline meter should have no trouble measuring quantities around a teaspoon. To be clear here, what I mean is that the meter can tell the difference between 1.000 gallons and 1.001 gallons. It will register a difference in volume between those two quantities of fluid.
> 
> So the pump is precise to 0.001 gallon, because it can measure and record volumes in increments that small.
> 
> But the pump is not accurate to one thousandth of a gallon:  
> \<more at Quora\>