# question on gas mileage **URL:** \ **Category:** Factual Questions\ **Created:** [December 17, 2005, 2:48am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014 "2005-12-17T02:48:14Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![sharkattack](https://avatars.discourse-cdn.com/v4/letter/s/76d3ee/32.png) [@sharkattack](https://boards.straightdope.com/u/sharkattack)\ **Post date:** [December 17, 2005, 2:48am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/1 "2005-12-17T02:48:14Z")
I drive a mazda MPV…about 10 years old. 6 cylinder. 3000 cc engine. I think its around a 50 litre tank. The question I have is this: If i fill the gas tank, how much will this affect my gas mileage (because of the weight of petrol) compared to if I fill it only halfway up? According to the dealer, I should be getting 7km to the litre. Thank you. ---
**Author:** ![FatBaldGuy](https://avatars.discourse-cdn.com/v4/letter/f/ecd19e/32.png) [@FatBaldGuy](https://boards.straightdope.com/u/FatBaldGuy)\ **Post date:** [December 17, 2005, 2:58am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/2 "2005-12-17T02:58:49Z")
The additional weight will have some effect on mileage, as will whether or not you carry passengers or other heavy items in your car. Given the overall weight of the car however, the effect should be negligible. ---
**Author:** ![lonesome\_loser](https://avatars.discourse-cdn.com/v4/letter/l/c68b51/32.png) [@lonesome\_loser](https://boards.straightdope.com/u/lonesome_loser)\ **Post date:** [December 17, 2005, 2:59am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/3 "2005-12-17T02:59:26Z")
The weight of the gas will have a minimal effect on your mileage. Other things, such as tire pressure and speed will provide you with much more savings. ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [December 17, 2005, 3:00am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/4 "2005-12-17T03:00:44Z")
> [@sharkattack](#): > > If i fill the gas tank, how much will this affect my gas mileage (because of the weight of petrol) compared to if I fill it only halfway up? Have you thought of trying an experiment and compare the results? ---
**Author:** ![wolf\_meister](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolf_meister/32/15202_2.png) [@wolf\_meister](https://boards.straightdope.com/u/wolf_meister)\ **Post date:** [December 17, 2005, 3:12am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/5 "2005-12-17T03:12:30Z")
Just to quantify matters: gasoline density = .75 that of H[sub]2[/sub]O = .75 kg per liter 50 liters of gasoline have a mass of .75 • 50 = 37.5 kilograms or 82.7 pounds ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [December 17, 2005, 3:13am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/6 "2005-12-17T03:13:28Z")
Half the tank is 25 liters. 4 liters weighs around 6 lbs (sorry for switching units). So you’re talking roughly 36 lbs of extra fuel weight. Your car weighs approximately 4000lbs, so the difference in fuel is about 1%. I doubt that’s going to make any real difference in gas milage. ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [December 17, 2005, 3:14am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/7 "2005-12-17T03:14:55Z")
This exact question has come up several times before. We have run through the numbers with the weight of the gas, increased fuel trips, etc, etc. The answer is it barely makes any difference at all and its certainly not worth it economically. The weight of the gas is trivial to the overall car weight and you mostly take weight into account only when you are accelerating. Most of the car’s energy is used to combat wind resistance and that doesn’t change here. Running some fuzzy numbers through my head including the weight of the gas versus the whole car and the wind resistance factor, the savings would have to be well less than 1%, ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [December 17, 2005, 3:16am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/8 "2005-12-17T03:16:12Z")
> [@Duckster](#): > > Have you thought of trying an experiment and compare the results? The factors are way too small to test outside of a laboratory under carefully controlled conditions. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [December 17, 2005, 3:23am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/9 "2005-12-17T03:23:08Z")
[This page](http://www.internetautoguide.com/car-specifications/09-int/1995/mazda/mpv/) says a 1995 Mazda MPV has a 74-liter tank. The density of gasoline at 20[sup]o[/sup] C is about 0.8 that of water, so a tankful will weigh about (0.8 \* 74) 59Kg, approximately the weight of a female passenger. The same page cites the MPV’s gross vehicle weight at 2352 Kg, about 40 times that amount. I’d say the effect was negligible though if it’s a concern, you can save weight by taking out the rear passenger seats when you don’t need them (if possible) as well as cleaning out any accumulated junk. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [December 17, 2005, 4:33am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/10 "2005-12-17T04:33:58Z")
And drop the spare tire too, you hardly ever use that. . . . 😉 ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [December 17, 2005, 4:45am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/11 "2005-12-17T04:45:52Z")
> [@CookingWithGas](#): > > And drop the spare tire too, you hardly ever use that. . . . > > 😉 As well as trim, extra seats, upholstery, extra gauges, carpet, glove compartment door, emblems, fat passengers, radio, speakers, bumpers, car jack, arm rests. I think we picked up a couple of percent just with that little bit. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [December 17, 2005, 5:48am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/12 "2005-12-17T05:48:02Z")
And why stop? At stops, you average 0 MPG. ---
**Author:** ![mcjunk](https://avatars.discourse-cdn.com/v4/letter/m/9dc877/32.png) [@mcjunk](https://boards.straightdope.com/u/mcjunk)\ **Post date:** [December 17, 2005, 6:06am UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/13 "2005-12-17T06:06:54Z")
This recently came up on Car Talk (comedy/automotive call-in show on public radio in the States). The answer was that the weight of gas is negligible, and carrying less gas might lower your total efficiency, since you’re going out of your way to the gas station more often. Another non-issue is the energy used to power the radio, heater, and lights. ---
**Author:** ![Mr.Slant](https://avatars.discourse-cdn.com/v4/letter/m/c57346/32.png) [@Mr.Slant](https://boards.straightdope.com/u/Mr.Slant)\ **Post date:** [December 17, 2005, 4:47pm UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/14 "2005-12-17T16:47:56Z")
Also, speeds above 70 KPH dramatically decrease gas mileage. I suggest driving to work in the slow lane at 70 KPH. You may get an added energy boost from the people who keep rear-ending you. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [December 17, 2005, 4:52pm UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/15 "2005-12-17T16:52:45Z")
Create a small black hole (the January 2006 issue of _Popular Science_ tells you how) moving in a circle between you and your workplace, and use the gravity assist for added efficiency. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [December 17, 2005, 5:36pm UTC](https://boards.straightdope.com/t/question-on-gas-mileage/336014/16 "2005-12-17T17:36:48Z")
> [@](#): > > he answer was that the weight of gas is negligible, and carrying less gas might lower your total efficiency, since you’re going out of your way to the gas station more often. A I would wag it will cost you money due to: More driving looking for gas stations. Having to buy gas at more expensive stations due to not having enough to get to the cheaper ones Having not enough gas in emergency situations, which can be very costly air space encourages condenstation which causes hesitation and other problems w/ the car. and perhaps having a greater air space in the tank has a bigger chance of an expolsion if there is a spark (like flight 800 center fuel tank after it got hit by the missile, I mean when a spark occured (I bet that spark was caused by a missile)).