# Why do gas pumps slow down before they stop? **URL:** \ **Category:** Factual Questions\ **Created:** [May 25, 2006, 4:36pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191 "2006-05-25T16:36:21Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Vashbul](https://avatars.discourse-cdn.com/v4/letter/v/3d9bf3/32.png) [@Vashbul](https://boards.straightdope.com/u/Vashbul)\ **Post date:** [May 25, 2006, 4:36pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/1 "2006-05-25T16:36:21Z")
Whenever you pump gas at a self-service filling station, there’s always that awful moment when you’re about 30 cents from your prepaid amount when the pump slows down to a crawl. Getting that last bit into the tank can seem to take forever, especially on a cold day. Why do they do this? Surely the technology exists for cutting off the flow of fuel instantly without accidently giving you a few drops more that you paid for (oh horror!). ---
**Author:** ![FatBaldGuy](https://avatars.discourse-cdn.com/v4/letter/f/ecd19e/32.png) [@FatBaldGuy](https://boards.straightdope.com/u/FatBaldGuy)\ **Post date:** [May 25, 2006, 4:38pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/2 "2006-05-25T16:38:43Z")
Instead of pre-paying, just pay at the pump with a credit/debit card. Then you don’t experience the slowdown. The pump just shuts off when the tank is full. ---
**Author:** ![Missy2U](https://avatars.discourse-cdn.com/v4/letter/m/3bc359/32.png) [@Missy2U](https://boards.straightdope.com/u/Missy2U)\ **Post date:** [May 25, 2006, 4:55pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/3 "2006-05-25T16:55:31Z")
Just don’t prepay. If you pay after you pump (even if you don’t fill the tank all the way up), that won’t happen - you have to make it stop - there’s no slowdown. ---
**Author:** ![vetbridge](https://avatars.discourse-cdn.com/v4/letter/v/b5e925/32.png) [@vetbridge](https://boards.straightdope.com/u/vetbridge)\ **Post date:** [May 25, 2006, 5:11pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/4 "2006-05-25T17:11:25Z")
> [@Missy2U](#): > > Just don’t prepay. However, as the price of gasoline increases, this option is rapidly disappearing (in my area, anyway). ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [May 25, 2006, 5:36pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/5 "2006-05-25T17:36:10Z")
> [@vetbridge](#): > > However, as the price of gasoline increases, this option is rapidly disappearing (in my area, anyway). For cash purchases, yes. But nearly all pumps in use these days have a credit/debit card reader built right in. You just swipe your card and start pumping. Some have an RFID reader that does the same thing with special RFID tags you keep your keyring. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 25, 2006, 6:35pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/6 "2006-05-25T18:35:32Z")
When you suddely stop a fluid flow in a pipe/hose you create a shock wave which can cause damage. So the faster the pump can shut off the flow the beefier the whole fuel delivery system would have to be. When prepay became into vouge, most pumps would allow full stream till the $ was reached then shut off, you usually got an extra 4-5 cents out of it - This is why I think they started the slowdown. ---
**Author:** ![Vashbul](https://avatars.discourse-cdn.com/v4/letter/v/3d9bf3/32.png) [@Vashbul](https://boards.straightdope.com/u/Vashbul)\ **Post date:** [May 25, 2006, 8:11pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/7 "2006-05-25T20:11:54Z")
> [@kanicbird](#): > > When you suddely stop a fluid flow in a pipe/hose you create a shock wave which can cause damage. So the faster the pump can shut off the flow the beefier the whole fuel delivery system would have to be. Wait a minute, that can’t be it. When your tank is full it always shuts itself off with a big “CHUNK” sound. Doesn’t need to slow down then. Which brings up a corollary question: how does it know? ---
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [May 25, 2006, 8:30pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/8 "2006-05-25T20:30:45Z")
> [@Vashbul](#): > > Wait a minute, that can’t be it. When your tank is full it always shuts itself off with a big “CHUNK” sound. Doesn’t need to slow down then. > > Which brings up a corollary question: how does it know? \<\\> [How does the gas pump shut off – Life After Coffee](http://www.lifeaftercoffee.com/2005/08/08/how-does-the-gas-pump-shut-off/) Of course you can always top off after the CHUNK, though it’s not advised. ---
**Author:** ![FatBaldGuy](https://avatars.discourse-cdn.com/v4/letter/f/ecd19e/32.png) [@FatBaldGuy](https://boards.straightdope.com/u/FatBaldGuy)\ **Post date:** [May 25, 2006, 8:32pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/9 "2006-05-25T20:32:28Z")
> [@Vashbul](#): > > Which brings up a corollary question: how does it know? There’s a backflow sensor built into the nozzle. As the tank fills, and a certain amount of trapped air & gasoline is pushed back up the narrow filler tube, the sensor is triggered and the pump shuts off. As for your original question about why it slows as it approaches the prepaid amount, I’m just guessing here, but we’re talking about 2 different methods of shutting off the pump. When you release the trigger, or the sensor triggers the shutoff, there is a manual valve in the pump handle that is closed immediately, shutting off the flow. When the computer has to tell the pump to stop pumping, it uses a different mechanism (I’m not sure what) that may not be as quick, or as precise, so it slows down to be more accurate. Again, just a WAG. ---
**Author:** ![Duke\_of\_Rat](https://avatars.discourse-cdn.com/v4/letter/d/ad7895/32.png) [@Duke\_of\_Rat](https://boards.straightdope.com/u/Duke_of_Rat)\ **Post date:** [May 25, 2006, 8:58pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/10 "2006-05-25T20:58:05Z")
Where I buy gas, you have to pre-pay and the pump doesn’t slow down or even stop at the pre-paid amount. If you go over, you have to go in an pay again. I’ve overshot a couple of times, maybe $.05, and had to go back into the store. I’d take a 30 second slowdown/stop any day. ---
**Author:** ![Vashbul](https://avatars.discourse-cdn.com/v4/letter/v/3d9bf3/32.png) [@Vashbul](https://boards.straightdope.com/u/Vashbul)\ **Post date:** [May 25, 2006, 9:04pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/11 "2006-05-25T21:04:26Z")
> [@kanicbird](#): > > When you suddely stop a fluid flow in a pipe/hose you create a shock wave which can cause damage. So the faster the pump can shut off the flow the beefier the whole fuel delivery system would have to be. Wait a minute, that can’t be it. When your tank is full it always shuts itself off with a big “CHUNK” sound. Doesn’t need to slow down then. Which brings up a corollary question: how does it know? ---
**Author:** ![Vashbul](https://avatars.discourse-cdn.com/v4/letter/v/3d9bf3/32.png) [@Vashbul](https://boards.straightdope.com/u/Vashbul)\ **Post date:** [May 25, 2006, 9:10pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/12 "2006-05-25T21:10:05Z")
(Ignore the above. I don’t know why that happened.) To Duke of Rat: At least they trust you enough to let you pump without prepaying. Not in Boston. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 25, 2006, 9:17pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/13 "2006-05-25T21:17:50Z")
> [@Vashbul](#): > > Wait a minute, that can’t be it. When your tank is full it always shuts itself off with a big “CHUNK” sound. Doesn’t need to slow down then The sound the shut off valve makes in air doesn’t matter. Also the nossel has a rubber tube on it, which absorbes some of that shock, and I don’t know how suddenly it actually shuts off the flow, in other words is that ‘clunk’ percise enough to end at a certain $ amount? ---
**Author:** ![kevininthe661](https://avatars.discourse-cdn.com/v4/letter/k/97f17d/32.png) [@kevininthe661](https://boards.straightdope.com/u/kevininthe661)\ **Post date:** [April 26, 2010, 11:33pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/14 "2010-04-26T23:33:25Z")
It’s purely to cheat you out of your money. Gas station owners can set the pump to slow down whenever they want it to. Stations used to do this around 5-10 cents early to avoid going beyond the pre-paid amount. NOW they set it much earlier - sometimes more than 50 cents before the pre-paid amount - because they want you to get impatient and leave early, thereby not getting all the gas you paid for and pumping up their profits. It’s solely done to aggravate you and get you to leave early. So don’t be surprised if pumps start to slow up even earlier. Complain to your station’s owner. Let them know we’re on to their scam. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 27, 2010, 8:36am UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/15 "2010-04-27T08:36:19Z")
> [@Vashbul](#): > > Whenever you pump gas at a self-service filling station, there’s always that awful moment when you’re about 30 cents from your prepaid amount when the pump slows down to a crawl. Getting that last bit into the tank can seem to take forever, especially on a cold day. Why do they do this? Surely the technology exists for cutting off the flow of fuel instantly without accidently giving you a few drops more that you paid for (oh horror!). When you’re driving and there’s a red light, do you wait until you get to the line and stop instantaneously, or do you slow to a stop? When you fill a drinking glass, do you pour the last bit as fast as the first? When a system has to stop accurately at a predetermined point, it’s just sensible design to make it slow down on the final approach. ---
**Author:** ![purplehorseshoe](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/purplehorseshoe/32/2996_2.png) [@purplehorseshoe](https://boards.straightdope.com/u/purplehorseshoe)\ **Post date:** [April 27, 2010, 3:18pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/16 "2010-04-27T15:18:35Z")
> [@Duke\_of\_Rat](#): > > Where I buy gas, you have to pre-pay and the pump doesn’t slow down or even stop at the pre-paid amount. If you go over, you have to go in an pay again. I’ve overshot a couple of times, maybe $.05, and had to go back into the store. I’d take a 30 second slowdown/stop any day. Great goshamighty, \*\*Duke of Rat \*\*- where are you? The Land of The Honest? What’s to stop you - and everyone else - from just driving off? What, they’re gonna call the cops on a nickel overage? Of course not, but I bet most people wouldn’t take the time to go back in, eventually bleeding the business owner. ---
**Author:** ![groman](https://avatars.discourse-cdn.com/v4/letter/g/73ab20/32.png) [@groman](https://boards.straightdope.com/u/groman)\ **Post date:** [April 27, 2010, 3:59pm UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/17 "2010-04-27T15:59:39Z")
I think the crucial difference here is where the cut off occurs. The auto-shut-off when full happens using a valve around the handle and propagates up to shut off the main pump. The auto-shut-off when out of money happens in the machine, so it slows down to let the hose empty into your tank. Maybe I am not being cynical enough, but these sorts of things tend to be strictly regulated and it is probably done in interest of fairness, rather than trying to cheat you out of your money. ---
**Author:** ![Vashbul](https://avatars.discourse-cdn.com/v4/letter/v/3d9bf3/32.png) [@Vashbul](https://boards.straightdope.com/u/Vashbul)\ **Post date:** [April 28, 2010, 1:20am UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/18 "2010-04-28T01:20:11Z")
> [@](#): > > When a system has to stop accurately at a predetermined point, it’s just sensible design to make it slow down on the final approach. I’d agree, but the reality (at least where I live) is that it slows down _way_ before it would need to as a way to either assure you get exactly what you paid for and no more, or to save potential wear on the pumping mechanism that a too-sudden shutoff might induce. Which is why I’m inclined to go with kevininthe661’s suspicion, which I didn’t think of, that it’s all about encouraging the impatient customer to quit filling early. ---
**Author:** ![BrotherCadfael](https://avatars.discourse-cdn.com/v4/letter/b/977dab/32.png) [@BrotherCadfael](https://boards.straightdope.com/u/BrotherCadfael)\ **Post date:** [April 28, 2010, 1:50am UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/19 "2010-04-28T01:50:15Z")
> [@kanicbird](#): > > so the **nossel** has a rubber tube on it I hear nossing, I see nossing, I know nossing! ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [April 28, 2010, 7:13am UTC](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191/20 "2010-04-28T07:13:18Z")
> [@Vashbul](#): > > Which is why I’m inclined to go with kevininthe661’s suspicion, which I didn’t think of, that it’s all about encouraging the impatient customer to quit filling early. Really? I didn’t think **kevininthe661** was even trying to be serious. [Next page](https://boards.straightdope.com/t/why-do-gas-pumps-slow-down-before-they-stop/358191.md?page=2)