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Yesterday my wife asked me that question, and I couldn't answer.

Consider a car, in a sunny day, and that is consumes x gallons per mile. Considering that everything is equal, except that it's traveling in a rainy day, but at the same temperature as the sunny day so that air density is the same.

Will the lower friction of the tires make it consume more or less fuel? And the fact that rain drops are falling over and in front of it ?

I answered that it'll consume more fuel, since friction is what makes car move and that the rain will act against it... but I'm not sure ?

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As long as your tires don't begin to slip, the change in the friction properties of the road surface won't make a difference. Running into a lot of raindrops would cause a slight drop in efficiency. I'm not sure how all that rain would affect airflow around the car, but that could cause a change in efficiency as well. Finally, I think there would be a slight adhesion between the tires and the wet road, which could also reduce efficiency a little bit. Holding off on making this an actual answer since I'd want it to be a bit more quantitative/precise for that. –  Kyle Jun 21 '13 at 15:20
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Depending on the engine type you could get a very slight boost in efficiency due to the increased humidity in the engine's air intake. Also, the increased humidity of the air while it is raining will decrease the density of air. This will result in a slightly lower dynamic pressure and thus slightly lower drag. –  OSE Jun 21 '13 at 16:29
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I have a degree in physics, but I'm speaking now based on my experience as a competitive race car driver. Wet roads don't have any added adhesion with the tire. In fact any amount of dampness reduces both adhesive and mechanical grip. Also, most engines are more efficient with damp air. I'm not sure why this is the case, and I find it confusing from a physics perspective, but experience supports it repeatedly. –  Colin K Jun 26 '13 at 1:05

4 Answers 4

up vote 23 down vote accepted

There is an additional loss of energy when driving through puddles on a wet road, because the tire treads have to exert work in order to eject water. One way to look at it is that the keeps trying to glide on top of the water, but is continuously sinking into it to meet the pavement, which is equivalent to driving slightly uphill.

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Yes, and afaik, tire companies put a lot of effort to decrease this effect by using an appropriate tread pattern. Well, they are after the handling in the first place, but nevertheless. Also I guess, this effect becomes dominant in the limit of really heavy rain. –  Peter Kravchuk Jun 21 '13 at 22:09
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Yes! I'd imagine this would be the dominant effect by far. All the water splashed to the side and kicked up into the air got that energy from your vehicle. I think this is a noticeable amount from personal experience, but as anecdotes go I may be imagining things. (Living in south California, I don't get the pleasure of driving in rain very often :P ) –  NeuroFuzzy Jun 21 '13 at 22:25
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Definitely. We tend to get a decent amount of standing water at the outside edge of the road and sometimes you can't avoid driving through it. The drag on the car is QUITE obvious. –  Loren Pechtel Jun 21 '13 at 23:17
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@NeuroFuzzy I agree with that being dominant in southern California, based on the time I lived and drove there. In other, non-desert parts of the country, roads are properly drained :P –  Chris White Jun 21 '13 at 23:49

Friction is indeed necessary for a car to push off the ground. However, it does not matter how much friction there is so long as your tires are not skidding. You only waste fuel if you put energy into spinning the wheels that is dissipated as heat before ever contributing to the car's motion. Otherwise, Newton's third law holds and the ground pushes you forward as much as you push back. So as long as you're not one of those drivers who keeps hitting the gas in thin puddles and hydroplaning, there shouldn't be much of a difference.

Of course, in order to keep from skidding you may very well have to travel slower, which can itself impact your efficiency (and is probably the dominant effect here - see this related question).

Air resistance is another thing entirely. If there are water droplets in the air, this increases its effective density, so you will spend more fuel pushing it out of the way. "Heavy" rain falls at about $25\ \mathrm{mm}/\mathrm{hr}$ ($1\ \mathrm{in}/\mathrm{hr}$). Elsewhere in that article we learn that large raindrops fall at a terminal velocity of $9\ \mathrm{m}/\mathrm{s}$. In order for these numbers to be consistent, the fractional volume of the air filled with raindrops must be $$ \chi_V = \frac{25\ \mathrm{mm}/\mathrm{hr}}{9\ \mathrm{m}/\mathrm{s}} = 8\times10^{-7}. $$ As the density of air is $1.2\ \mathrm{kg}/\mathrm{m}^3$, while water is more or less by definition $1000\ \mathrm{kg}/\mathrm{m}^3$, we see the fractional change in air density, assuming constant gas density, is something like $$ \chi_\rho = \frac{\chi_V(1000\ \mathrm{kg}/\mathrm{m}^3)}{1.2\ \mathrm{kg}/\mathrm{m}^3} = 6\times10^{-4}. $$ So yes, rain will make it harder, by a very small amount, to push through the air.


Addendum: There are many other factors worth considering, many of which have more interesting and complicated physics than the simple stuff I felt like analyzing. Kyle points out in a comment that water can be adhesive, so it can be harder to drive through (this is a less extreme example of driving through tar). Displacing water also costs something, as noted in an answer by kaz. Both of these are probably larger effects than increasing the effective air mass. Frank Presencia Fandos's answer mentions an even larger concern, engine efficiency based on airflow (and readiness to participate in combustion), which is an engineering problem too complicated for a physicist like me. OSE's comment also points out an implicit assumption I made above that should be said explicitly: I'm comparing $100\%$ humidity weather with and without rain. If you want to compare to dry weather, the effect goes the other way, mainly because a molecule of nitrogen is more massive than a molecule of water.

In the end, though, I'm willing to bet the biggest effect is still the fact that most drivers slow down in heavy rain, since fuel efficiency is highly dependent on speed.

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What about all that surface water that you're displacing? –  Glen The Udderboat Jun 21 '13 at 19:58
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I'll take that bet! There are many "MPG enthusiasts" on the Internet. Here's one discussing my previous comment as well: How about Rain; will it hurt my mileage? (Which is then discussed here.) –  Glen The Udderboat Jun 22 '13 at 8:43

I would like to consider just one aspect of this question. I am not sure that, as Chris White put it, "it does not matter how much friction there is so long as your tires are not skidding." The coefficient of rolling friction does depend on the surface, so I guess it is somewhat different in rain. I guess rolling friction losses are somewhat important (http://www.consumerenergycenter.org/transportation/consumer_tips/vehicle_energy_losses.html ), even for driving wheels.

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Yes, I thought about it, like those green tires –  woliveirajr Jun 21 '13 at 18:12

I would say yes, I'm in a hurry, but these are the 2 main reasons I can say right now:

  • Air density is NOT the same, as Chris White noted.

  • The water heat capacity and thermal conductivity is different than the air. I can only wildly guess now as there are too many factors involved, but this will likely make it consume more fuel. The engine needs to be hot to deliver it's best and metal is a good conductor. Air not so much, but then water is. So my guess is, even if at the same temperature, the water will make the whole car leak more thermal energy, thus making it consume more energy (fuel). Besides, if you want to keep the car's inside temperature at a different one than the outside one, this will affect it as well.

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And what is the air density then? –  Peter Kravchuk Jun 21 '13 at 22:27
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That comma, sorry @PeterKravchuk –  Francisco Presencia Jun 21 '13 at 22:48
    
Thanks for the clarification) –  Peter Kravchuk Jun 21 '13 at 22:51

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