Does rain improve gas milleage?

[burritos]

Apologies if this has already been discussed. Intuitively the friction of rain hitting the car and water on the ground if anything diminishes fuel efficiency. However, when there's rain, people drive slower. On the highway if it's raining pretty hard, the speed goes into the sweet spot which is 40-50 mph. This improves the fuel efficiency. On a 50 mi trip, I noticed that the mpg was 47 cause we were flying at 70 mph. On the way back, there was a strong downpour slowing the traffic to the 40-50's mph. The final mpg was 51.

[efusco]

<div class='quotetop'>QUOTE(burritos @ Sep 22 2007, 02:40 PM) [snapback]516261[/snapback]</div>

Apologies if this has already been discussed. Intuitively the friction of rain hitting the car and water on the ground if anything diminishes fuel efficiency. However, when there's rain, people drive slower. On the highway if it's raining pretty hard, the speed goes into the sweet spot which is 40-50 mph. This improves the fuel efficiency. On a 50 mi trip, I noticed that the mpg was 47 cause we were flying at 70 mph. On the way back, there was a strong downpour slowing the traffic to the 40-50's mph. The final mpg was 51. [/b]

No, rain definately has a negative impact. There was probably a strong tail wind plus the slower speeds will definately help you. But all other conditions being equal rain will negatively impact FE.
http://privatenrg.com/RainyRoad.htm#How_about_Rain

[donee]

Hi Burritos,

High Humidity, cloud covered dry conditions tend to improve fuel economy. If you had any of that during your trip and then it started to rain, that could be a partial explaination.

Humidity reduces the air density, and results in less air drag.

Rain definately decreases fuel economy. Especially if the rain is blowing into the radiator. Besides resulting in more fuel burn to keep the engine warm, the tires act like pumps to push the water aside. This increases rolling resistance at highway speeds due to the dynamic forces of the pumping actions.

[oms2burn]

I recently experience similar results to Burritos. I chalked it up to less friction (tires slip, so therefore less friction). In retrospect, the tires pumping the water makes more sense. I suspect it just slowed me (and others) down just enough to help gas mileage. Interesting thread.

[burritos]

<div class='quotetop'>QUOTE(efusco @ Sep 22 2007, 02:46 PM) [snapback]516262[/snapback]</div>

No, rain definately has a negative impact. There was probably a strong tail wind plus the slower speeds will definately help you. But all other conditions being equal rain will negatively impact FE.
http://privatenrg.com/RainyRoad.htm#How_about_Rain
[/b]

Right but is your gas mileage better going at 70 mph in perfect conditions or 45 mph in rain.(on the same road)?

[efusco]

<div class='quotetop'>QUOTE(burritos @ Sep 22 2007, 06:56 PM) [snapback]516350[/snapback]</div>

Right but is your gas mileage better going at 70 mph in perfect conditions or 45 mph in rain.(on the same road)? [/b]

Go to the link I provided above, download the Prius Simulator for Windows and drop in the numbers. I suspect the 45 will be a bit better, but probably not a lot.

[qbee42]

<div class='quotetop'>QUOTE(burritos @ Sep 22 2007, 07:56 PM) [snapback]516350[/snapback]</div>

Right but is your gas mileage better going at 70 mph in perfect conditions or 45 mph in rain.(on the same road)?
[/b]

Your logic is correct that there will be some horrible differential equation describing the mileage sweat spot in relation to rain and speed. There is no one simple answer. Driving slowly in rain will yield better mileage than really fast in dry conditions. Exactly where the cross-over occurs depends on the road, tires, temperature, wind, and the amount of rain. It wouldn't surprise me for you to get better mileage at 45 in the rain than 70 in dry conditions, at least in some conditions.

Tom

[alexstarfire]

<div class='quotetop'>QUOTE(donee @ Sep 22 2007, 04:19 PM) [snapback]516268[/snapback]</div>

Humidity reduces the air density, and results in less air drag.[/b]

Are you sure about that? I haven't checked out the link yet, but I will once I've posted. Intuition tells me that humidity is more water in the air and water is heavier than air. So wouldn't that mean that the air is more dense and in result have more wind resistance on the car?

[hyo silver]

It's not just the gas mileage we have to worry about. Apparently, rain can get in the engine and really screw things up.

[Rae Vynn]

<div class='quotetop'>QUOTE(hyo silver @ Oct 4 2007, 08:21 AM) [snapback]521174[/snapback]</div>

It's not just the gas mileage we have to worry about. Apparently, rain can get in the engine and really screw things up.
[/b]

Oh oh...