# How many MPG would a car get from using water as fuel?

**URL:** <https://boards.straightdope.com/t/how-many-mpg-would-a-car-get-from-using-water-as-fuel/506221>\
**Category:** Factual Questions\
**Created:** [August 12, 2009, 8:44pm UTC](https://boards.straightdope.com/t/how-many-mpg-would-a-car-get-from-using-water-as-fuel/506221 "2009-08-12T20:44:40Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![Bad\_Astronaut](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bad\_Astronaut](https://boards.straightdope.com/u/Bad_Astronaut)\
**Post date:** [August 13, 2009, 10:08pm UTC](https://boards.straightdope.com/t/how-many-mpg-would-a-car-get-from-using-water-as-fuel/506221/21 "2009-08-13T22:08:58Z")

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> [@billfish678](#):
>
> Not neccessarily.
> 
> Basically, you MIGHT have a chemical compound that has stored energy and releasing that energy is done by mixing it with water. That energy is then expressed as the production of Hydrogen and Oxygen. You can extract THAT energy for useful purposes by burning it in an IC engine or a fuel cell.

That wouldn’t change the fact that virtually all of the input water is recoverable.

The difference would be that we would then care about mileage in terms of the magical compound, not the water.

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**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\
**Post date:** [August 13, 2009, 11:01pm UTC](https://boards.straightdope.com/t/how-many-mpg-would-a-car-get-from-using-water-as-fuel/506221/22 "2009-08-13T23:01:46Z")

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> [@Bad\_Astronaut](#):
>
> That wouldn’t change the fact that virtually all of the input water is recoverable.
> 
> The difference would be that we would then care about mileage in terms of the magical compound, not the water.

All the water produced in normal internal combustion engine is “recoverable”…but nobody does it, because…its WATER and nobody cares.

Water used in old fashion steam locomotives was recoverable too…but nobody generally bothered.

If your magic compound mixed with water produces hydrogen, or hydrogen and oxygen to burn, you COULD recover the steam, but why the hell do it? waters like cheap and easy to get more of.

Therefore you WILL be “using” water along with the compound.

And we’ve covered the using up of the compound like 4 or 5 times now.

At first glance, using lithium to react with water to produce hydrogen to burn doesnt look too horrible energy wise. WAG is a tenth MPG that of gasoline without crunching the numbers, but still better than lithuim ion batteries on a per pound basis.

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**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\
**Post date:** [August 14, 2009, 5:33am UTC](https://boards.straightdope.com/t/how-many-mpg-would-a-car-get-from-using-water-as-fuel/506221/23 "2009-08-14T05:33:51Z")

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> [@scr4](#):
>
> But the fact is, hydrogen + oxygen (as separate gases) _contains_ more energy than the water. So if you have a magical way to do it without using energy, what you have is a magical way of getting energy out of nothing. If we assume that’s possible, Laws of Thermodynamics are out of the door, and _anything_ is possible. Perpetual motion and all that.

[voice=Homer]  
On this message board, we obey the Laws of Thermodynamics.  
[/voice]

Surprised no one else said it first

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