# Gas Weight and Gas Mileage **URL:** \ **Category:** Factual Questions\ **Created:** [May 25, 2006, 6:03pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214 "2006-05-25T18:03:39Z")\ **Posts on this page:** 20\ **Page:** 1
**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, 6:03pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/1 "2006-05-25T18:03:39Z")
My husband has been telling me that with gas prices as high as they are, he won’t put more than $5 at a time in the gas tank because it just adds weight, which negatively affects gas mileage. This results in me having to go to the gas station like every day. This also results in me being very annoyed. He is telling me the truth? If a gallon of gas weighs 7 pounds, and we have a 15 (?) gallon tank, does the difference of 91 pounds really make that big of a difference in the mileage we get?? ---
**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:** [May 25, 2006, 6:34pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/2 "2006-05-25T18:34:53Z")
Probably not near as much difference as the extra pulling in and out of the gas station and the additional engine start do. It sounds penny wise and pound foolish to me. ---
**Author:** ![iamthewalrus\_3](https://avatars.discourse-cdn.com/v4/letter/i/258eb7/32.png) [@iamthewalrus\_3](https://boards.straightdope.com/u/iamthewalrus_3)\ **Post date:** [May 26, 2006, 12:17am UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/3 "2006-05-26T00:17:15Z")
You should just counter with the fact that, since gas prices are increasing faster than inflation, you’re better off buying a full tank whenever you can. It’s like your own little gasoline futures investment. ---
**Author:** ![Mops](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mops/32/16494_2.png) [@Mops](https://boards.straightdope.com/u/Mops)\ **Post date:** [May 26, 2006, 12:39am UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/4 "2006-05-26T00:39:32Z")
> [@Gary T](#): > > Probably not near as much difference as the extra pulling in and out of the gas station and the additional engine start do. It sounds penny wise and pound foolish to me. Not to mention any detours incurred in visiting the gas station more often. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [May 26, 2006, 12:46am UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/5 "2006-05-26T00:46:29Z")
> [@iamthewalrus(:3=](#): > > You should just counter with the fact that, since gas prices are increasing faster than inflation, you’re better off buying a full tank whenever you can. It’s like your own little gasoline futures investment. Love it. And probably true. What does YOUR time cost. Putting gas in the car every day costs you. > [@iamthewalrus(:3=](#): > > This also results in me being very annoyed. I don’t blame you. Also, I never let my car get below a quarter of a tank or so. A lot of that has to do with where I live. But you never know if you will need the car and don’t have enough gas. Just fill it up. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [May 26, 2006, 12:57am UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/6 "2006-05-26T00:57:37Z")
> [@Missy2U](#): > > If a gallon of gas weighs 7 pounds, and we have a 15 (?) gallon tank, does the difference of 91 pounds really make that big of a difference If gasoline were 7lbs/gal, then 15 gallons would be 105 lbs. In fact, its density (at 22 degrees C) is around 737 kg/m[sup]3[/sup], which converts to 6.15 lb/gal. This means that 15 gallons weighs around 92 pounds - close to what you suggest. So you’re talking about something like 4% of the car’s weight. The effect of this on gas mileage (as others have noted) if not actually negligible should be really quite small. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [May 26, 2006, 1:03am UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/7 "2006-05-26T01:03:22Z")
He is causing problems, like condensation in the tank, as well as forcing one to fill up all the time, which might mean you have to fill up at a station that charges more. ---
**Author:** ![R.P.McMurphy](https://avatars.discourse-cdn.com/v4/letter/r/7993a0/32.png) [@R.P.McMurphy](https://boards.straightdope.com/u/R.P.McMurphy)\ **Post date:** [May 26, 2006, 1:04am UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/8 "2006-05-26T01:04:56Z")
One of our idiot founding fathers (I think it was Ben somebody) made the absurd observation that “time is money”. Not that I have to believe it but some people do. What’s your husband’s time worth? Not much, I’d have to assume. :smack: :smack: :smack: It’s like driving 20 miles out of your way to save 3 cents a gallon. ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [May 26, 2006, 1:22am UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/9 "2006-05-26T01:22:40Z")
> [@enipla](#): > > Also, I never let my car get below a quarter of a tank or so. I’ve had mechanic friends recommend this. There can be contamination in gas & in your gas tank (mostly water that condenses inside the tank), and most of this floats on top of the gas. Keeping the tank at least 1/4 full means these contaminents just stay at the top, and don’t ever get into the engine. ---
**Author:** ![R.P.McMurphy](https://avatars.discourse-cdn.com/v4/letter/r/7993a0/32.png) [@R.P.McMurphy](https://boards.straightdope.com/u/R.P.McMurphy)\ **Post date:** [May 26, 2006, 1:28am UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/10 "2006-05-26T01:28:21Z")
> [@t-bonham@scc.net](#): > > I’ve had mechanic friends recommend this. There can be contamination in gas & in your gas tank (mostly water that condenses inside the tank), and most of this floats on top of the gas. Keeping the tank at least 1/4 full means these contaminents just stay at the top, and don’t ever get into the engine. I think that this goes along with the advice to never let a car with fuel injection run out of gas. When you run out of gas any residual contaminents in the tank get sucked into the gas line and can clog the fuel injectors. With the old carburators it wasn’t such a concern but modern engines aren’t as tolerant. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [May 26, 2006, 12:44pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/11 "2006-05-26T12:44:47Z")
> [@t-bonham@scc.net](#): > > There can be contamination in gas & in your gas tank (mostly water that condenses inside the tank), and most of this floats on top of the gas. No - water is denser than gasoline and thus will be found at the bottom of the tank. ---
**Author:** ![gigi](https://avatars.discourse-cdn.com/v4/letter/g/a587f6/32.png) [@gigi](https://boards.straightdope.com/u/gigi)\ **Post date:** [May 26, 2006, 1:02pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/12 "2006-05-26T13:02:28Z")
> [@tschild](#): > > Not to mention any detours incurred in visiting the gas station more often. And the $1 cappucinos “required” by a stop at our neighborhood station. ---
**Author:** ![vetbridge](https://avatars.discourse-cdn.com/v4/letter/v/b5e925/32.png) [@vetbridge](https://boards.straightdope.com/u/vetbridge)\ **Post date:** [May 26, 2006, 1:25pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/13 "2006-05-26T13:25:56Z")
> [@Xema](#): > > No - water is denser than gasoline and thus will be found at the bottom of the tank. Really? As a kid, I worked at a gas station. We dipped the tanks with a long pole to measure how many inches were present. After initial measurements were done, a water sensitive paste was applied to the area where gas/air interfaced in order to see how many inches of water was present. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [May 26, 2006, 1:58pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/14 "2006-05-26T13:58:00Z")
> [@vetbridge](#): > > Really? As noted above, gasoline’s density is about 74% of water’s. As a routine check, pilots “sump” their fuel tanks before flight. This involves draining a small amount of fuel from the lowest point of each tank into a clear container. If any trace of water is present, it will show here. ---
**Author:** ![Max\_Torque](https://avatars.discourse-cdn.com/v4/letter/m/e9bcb4/32.png) [@Max\_Torque](https://boards.straightdope.com/u/Max_Torque)\ **Post date:** [May 26, 2006, 1:59pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/15 "2006-05-26T13:59:08Z")
So, does he also refuse to carry passengers around? A single passenger would weigh more than a full tank of gas. Hey, maybe you can use this line of logic to convince him to lose weight (assuming he needs to)! “Honey, if you lose those extra 30 pounds, you could put an extra 4 1/2 gallons of gas in the tank!” ---
**Author:** ![muttrox](https://avatars.discourse-cdn.com/v4/letter/m/a8b319/32.png) [@muttrox](https://boards.straightdope.com/u/muttrox)\ **Post date:** [May 26, 2006, 2:24pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/16 "2006-05-26T14:24:46Z")
But a few 10-gallon standalone gas tanks. Go to the gas station, fill 'em up. Store them safely at home. When your husband needs gas, he gets it from there, rather than having to drive to the gas station every time. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [May 26, 2006, 2:43pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/17 "2006-05-26T14:43:01Z")
IIRC (and please correct me if I’m wrong), some cars have fuel pumps cooled by the gas in the tank, so it’s not a good idea to let the gas run low and keep it there. Also you’re losing some gasoline vapor every time you open the gas cap. And you’d be starting the engine more often, which increases wear on the engine. There are numerous other ways of saving gas that are more effective. Like accelerating gently, letting off gas early when approaching a red light or a turn, and above all, obeying the speed limit. And of course, combining multiple trips into one, and eliminating unnecessary trips (like a daily trip to the gas station!). ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [May 26, 2006, 4:10pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/18 "2006-05-26T16:10:11Z")
> [@scr4](#): > > IIRC (and please correct me if I’m wrong), some cars have fuel pumps cooled by the gas in the tank, so it’s not a good idea to let the gas run low and keep it there. > > Also you’re losing some gasoline vapor every time you open the gas cap. And you’d be starting the engine more often, which increases wear on the engine. > > There are numerous other ways of saving gas that are more effective. Like accelerating gently, letting off gas early when approaching a red light or a turn, and above all, obeying the speed limit. And of course, combining multiple trips into one, and eliminating unnecessary trips (like a daily trip to the gas station!). Well you right and wrong. Fuel injected cars have electric pumps that are cooled by the fuel flowing inside them. So the level inside the tank is pretty much immaterial as long as the pump does not run for an extended time with no fuel there is no problem. **vetbridge** Either your memory is starting to go, or you guys did it wrong. At my gas station we put the paste on the bottom of the stick and quite often found water, since our tanks leaked. **Spartydog, [t-bonham@scc.net](mailto:t-bonham@scc.net)** I have never seen an good, believable cite that for a gasoline powered car that running out of gas was overly harmful. As far as contamination goes, there is a sock type filter to keep the big pieces out of the system and a huge very fine filter between the pump and the injectors. As far as the OP goes, $5 would not get me to work in some cars. Empty your trunk, set the air pressure in your tires to the max recommended by the car maker, use your gas pedal lightly and get off the throttle early when you see a red light ahead. These will all save more gas than not filling your tank. ---
**Author:** ![vetbridge](https://avatars.discourse-cdn.com/v4/letter/v/b5e925/32.png) [@vetbridge](https://boards.straightdope.com/u/vetbridge)\ **Post date:** [May 26, 2006, 4:22pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/19 "2006-05-26T16:22:31Z")
> [@Rick](#): > > **vetbridge** Either your memory is starting to go, or you guys did it wrong. At my gas station we put the paste on the bottom of the stick and quite often found water, since our tanks leaked. Either one is possible. This was circa 1976.😉 ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [May 26, 2006, 4:38pm UTC](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214/20 "2006-05-26T16:38:05Z")
Is there any truth to the notion that a fuller gas tank will help the pump get fuel to the engine via pressure? That never made much sense to me, and the notion that it would save fuel (in terms of not having the car’s pump work as hard,) versus the gas spent by lugging the extra weight seemed odd to me, but this is as good a time as any to mention that. [Next page](https://boards.straightdope.com/t/gas-weight-and-gas-mileage/358214.md?page=2)