anyone heard of this gas mileage theory

[no_to_war_economy]

you always fill up at half a tank to increase miles per gallon

can anyone prove.disprove this thesis

[whistle]

1. Not valid, this was an urban legend that came about in the 1970's

...as a way to get people to not notice the cost of a real fill-up (when tanks were nearly empty) and also when gas was being rationed to just 10 gallons per visit.

[papau]

2. It is slightly true - the more empty the tank the larger amount of gas is

lost due to becoming gas fumes and drifting away.

It is also the reason for the new gas cap covers and the cover on the gas pump metal going into the tank.

I doubt that you would notice the dif.

[MindPilot]

12. This myth is busted.

Since the early 70's cars have vapor recovery systems; fuel vapor is trapped in a canister and purged into the engine. The only time any fuel vapor at all escapes is when the gas cap is off.

It is however a good maintenance procedure to keep the tank full because a lower fuel level exposes more area for condensation to occur. And as we all know, moisture in the fuel is a bad thing.

[papau]

19. Thanks for the info - as to water, I thought that was a myth as small

amounts of water were said to improve performance - at least in my memory of a talking head on TV.

very small amounts of water, of course, does not mean pouring a glass of water into the tank - but I thought there was some data out there re moisture that showed it to be good at very small amounts.

But I am zero for one today in the pontificating department.

I think I will quit while I am behind!

:-)

[MindPilot]

23. I can pontificate on that one too!!

Adding water to the fuel mixture--not to the fuel itself--can increase performance and was used in aircraft and racing applications. The idea is that water, containing hydrogen and oxygen, two substances that seriously aid combustion will augment the energy stored in the fuel.

The problem is the systems don't give enough performance increase to justify their weight and complexity.

And in a regular car engine, the combustion temperatures never get high enough to break the bond between the hydrogen and oxygen, so literally all you're doing is running water through your engine.

[Walt Starr]

3. Not true, but still a good idea

For emergency reasons, letting your gas tank drop below half is not a good idea. In an emergency you may have to leave your immediate area fast and may not have time to fill up or gas may not be available.

[papau]

5. Ok Mr Whistle and Mr Star - do you mean the gov back in the 70's

was not telling the truth!

:-(

[Walt Starr]

6. To put it quite bluntly

they were not telling the truth. They wanted you to fill up your tank more regularly so as not to notice the real cost of a tank of gas.

[papau]

10. OK - now I must do a memory dump - and it sounded so plausible

- at least at the edges.

Of course the gas tank for the yard machines never has gas evaporate over time - but that I attributed to the great gas cap!

[The Night Owl]

4. Some answers...

http://www.faqfarm.com/Q/Does_a_car_use_more_gas_when_the_tank_is_below_half_full

[no_to_war_economy]

9. i like your link, good info thanks

see people DO get along on DU

hug?

[SidDithers]

7. I'd suggest the opposite...

If you always fill up at 1/2 tank, then you're always driving around with an average of ~ 3/4 tank of gas. With a 15 gallon tank, the avg weight would be around ~ 90 lbs.

If you let your tank go to empty and then fill up, you're driving around with an avg of 1/2 tank, ~60 lbs.

If you only ever fill 1/2 a tank (the bottom half) then you're driving around with avg of 1/4 tank, ~ 30 lbs.

All things being equal, a lighter car uses less gas, so a fuller tank would be less efficient than an emptier tank.

Or something like that... :)

Sid

[papau]

11. I like your point - net-net the car will have less miliage seems a solid

conclusion.

[MindPilot]

13. That makes much more sense but the difference would be negligible.

You probably use more gas running your stereo or driving with the windows down.

[Lex]

16. You use gas by running your stereo?

Wow.

[mccoyn]

17. A very very small amount.

It takes electricity, which drains the battery, which increases the electrical resistence for the alternator, which increases the torque your alternator uses, which reduces miles per gallon. Seriously small amount though.

I've heard that rolling your windows down on some of the more aerodynamic cars uses more gas (by causing drag) than running the air conditioner.

[MercutioATC]

21. I've heard that below 40mph, roll down the windows...above 40, use the AC.

...as a general rule.

[badger1080]

15. airlines

I'd always assumed the reason airplanes don't completely fill their tank was to reduce weight, thereby conserving fuel.

[newyawker99]

20. Hi badger1080!!

Welcome to DU!! :toast:

[MercutioATC]

22. It has more to do with fuel prices at different airports...

They'll fill up if the price is a little less someplace.

What's funny is when they miscalculate and ask to fly lower/take a detour/dump fuel because they took on so much fuel that they're too heavy to land.

:eyes:

[jayfish]

8. I Would Think The Opposite IS True.

More fuel=more weight. More weight=lower MPG. Although it's probably negligible. The theory you are speaking of was probably circulated by Arbusto or Halliburton.

Jay

[mccoyn]

14. Some family sayings.

My grandpa always said its just as easy to keep the tank more than half full as it is to keep it less than half full. My mom always said it was impossible.

[heidler1]

18. In the colder states it is probably a good idea to keep your tank pretty

full they say it will cut down on the condensation on the inside of your tank and lower the chance of a gas line freeze up. I read this several times years ago. Now that some gas has corn juice added it's also probably wise to use those brands in winter to avoid the problem at least part of the time. Alcohol absorbs water.