Effect of wet roads on efficiency: estimated 14.5% MPG drop
 

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We're all familiar with the effects of temperature on fuel economy. you may have also heard that smooth roads produce better numbers than bumpy ones. concrete's better than asphalt, etc.

I've often wondered what impact a wet road has on fuel consumption. the most obvious drag you feel is when you hit a deep patch of standing water, but there must also be a continuous low-level drag as the tires "part" the water and throw a constant spray behind them. (not to mention the added electrical load of lights & wipers if it's raining.)

I've looked around a few times for info on this, and have only come up with one page.

The claims are pretty astounding - with the result that i'm somewhat skeptical. but I haven't found any competing information on which to base (or reduce) my skepticism.

The claim is:

That's a mileage drop of 14.5% in the wet. The calculation is for a Prius at 65 mph at 37F.

(Note: this is a calculation, not an observed/tested result. but I've read quite a bit on this guy's site (prius aficionado), and he's clearly on the ball. the calculation is from a complex palm application that calculates energy use based on a multitude of adjustable variables.).

RH77:

Excellent post as usual MetroMPG. The effect of wet roads has to significantly decrease mileage, but that all depends on tire choice. A while back, a thread was started to discuss the effect of tire resistance and fuel economy. The lower rolling resistance of tires (perhaps like the Goodyear Brand "AquaTread") might be pretty good at pushing water out of the way, but might be crummy on dry pavement (and how much energy does that take to push the water). Long story short, where the rubber meets the road could significantly effect economy. A search was on, BUT...

Tire companies are not required, and do not test, the Coefficient of Resistance on wet or dry roads -- so no new data was gathered (and no government agency or University/Institution stepped up to the plate to perform new test). I say "new" because in early 2003, an indepenent agency, hired by a bill by the Great State of California, performed tests on tire resistance. GreenSeal, a not-for-profit environmental group has made the data public on the Internet (a good article too, by the way).

My next set of tires will be based on this testing. Hopefully new data comes out, as some tire models are outdated. I think the only company that performs this testing is located in the Rubber City (Akron, Ohio -- where I'm from originally) at a premium (Smithers Inc.)

I've even tried the TireRack, who has their own test track, but they have no scientific way to test resistance.

RH77

SVOboy:

I think the best thing for rain would be narrow tire width, as it has less water to push out of the way, also makes it handle better in the rain.

14%mpg loss is actually a pretty low
 

I know the original post is 2 years old now, but since there aren't many on this topic I am going to reply.

I am driving a 2001 Civic LX and usually get about 33-34 mpg - this in South California where it's usually pretty dry. It's been pouring rain for a couple of days now, and the difference in MPG is monumental - down to about 26-27 (which would mean at least 20% loss). My tires are Pirelli Cinturato P5, all weather.

Welcome to the site ncracker.

Your statement sounds a bit off the deep end. You are talking about a ~25% decrease in FE from wet roads. I find that pretty hard to believe. Maybe from the crappy traffic, but not from the road being wet.

In the LA area there have been a lot of rain in the las two days, so the amount of water on the ground could be affecting his mileage. Some areas are even flooded. My trip to work usually averages about 28MPG and today it was 26MPG and the road here was just moderately wet with a light drissle of rain. The 5 freeway drains well, so the drop seemed right.

My question is this:

Water has a lubricating effect. If the road is wet, but not wet enough for water to actually stand on the surface, needing to be moved, does this mitigate losses in FE due to rain?

I would think it would actually increase FE in this scenario, since water has the lubrication effect, which would reduce frictional losses between the tires and the road.

Thoughts?

Some have also argued that the high humidity and water in the air produces a water injection effect that allows ignition timing to be advanced and increases efficiency too... Too many variables IMO.

My historical data shows a 12% reduction in FE when it's wet. I record it as "wet" when I would rather not be out in that weather, or would choose to take an umbrella, or right after a rain when water is standing on the roads. If it's just misting but the roads are dry, I count that as dry.

My data also shows an 8% gain when it's humid. Comparing only morning commutes, and calling it "humid" when there is dew on the car or fog in the air.

My recent experience her in Northern Michigan with a recent very rainy period (seemed like weeks) showed only about a 5% drop. (2mpg's). Those who are running hyper-inflated tires are less prone to big drops.

It seems the lubricating effect would be a reason for less mpg's, as the tire is actually slipping a very little with each rotation (like in the winter only not as bad) making for less efficiency. You are not travelling as far for a given rotation.

I doubt there's any slipping going on. My commuting distance traveled doesn't change between rainy days and dry days.

I've been running "hyper" inflated tires for the duration of the data collection above. (nearly 3 years of daily commuting)

Hello -

Today I got 49.4 MPG on the way to work. I took an alternate route that swapped a few freeway miles for a few city miles, but avoided the monster traffic jams that I was seeing on sigalert.com . I can normally get sub-60 MPG. I think 57 MPG is about "normal" for my regular route. That would mean :

49.4 / 57 = 0.866666 => 86.7% => a 13.3% drop in MPG.

I am not making any scientific claims here, but I think I am seeing "ballpark same" results as the study.

CarloSW2

Anything more than the friction necessary to maintain tractive response between the tire and the road is wasted as heat. The water acts as a coolant and friction modifier, reducing the actual Cf between the rubber and the road, as it were. Water which as wet the road to a state of equilibrium will be in small pockets formed in the asphalt, and in such small quantities, water can exhibit qualities of a solid, especially if approached at high velocities, due to the surface tension and overall density. This would suggest that in addition to having a lubricant property, it also "smooths out" the surface of the driven road, which we already know is of benefit to fuel economy, but not tractive motion.

Over the past two weeks I've had some experience with wet roads (some days more than others) and I've seen about a 10% drop in mpg on the days I specifically tracked my mpg for that day. On my commute to work (same route every day) I average in the 65 mpg range at this time of year. Twice last week on wet roads with ALOT of standing water I averaged 58 and 59 mpg (10-11% drop).

In my Prius ANY water dramatically slows the car...certainly much more than I experienced in my 4runner.

I know my EOC distance is not as good on wet roads. Probably the biggest reason for my difference in mpg's. Anyone up for some coastdown testing? Another thing is the humidity and a lower pressure associated with a rain pattern contributes to higher density altitude, which means less efficient engine.

On my commute, I get my best fuel economy in the rain because traffic goes much slower.

On the open road, rain and wind (often seen together) are noticeably detrimental, according to the ScanGauge.

Obviously, you don't live in/near Harrisburg, PA or NY, NY. I can't vouch for other areas, but in those two places specifically, when it rains, even if you can't see 10 feet in front of you, traffic seems to speed up. It's like they know the cops aren't around because they can't do anything even if they were, and they go nuts!

Ah, but some would argue the lower air density = less aerodynamic drag. :)

And I've read some people swear they get better MPG at altitude, with the less dense intake charge depowering the engine, requiring a wider throttle opening, reducing pumping losses and improving efficiency. Same effect could happen with low vs. high pressure at the same altitude (assuming OBD computer controlled mixture).

And I completely agree about the shorter coasting distance on wet roads.

I've noticed a slight drop in FE (10%) when it's raining. This may be b/c then I drive in gear a little more often, less coasting, especially in traffic. But then I drove slower, and use even more engine braking.

I've also been thinking that effect of raindrops on the car. When a car is going at a constant speed and is hitting drops of rain (whose velocity vector is usually orthogonal to the car's direction of movement - I'm assuming no wind), then each of those drops gets accelerated to the car's speed. When you add up the total weight of all drops over a certain distance or time duration, it's like accelerating a large mass of water.

Piwo - This falls back on something I thought about, as well.

Since you're hitting all those rain drops, each one having a very small impact which occurs at a random interval, each one of those rain drop impacts is effectively slowing down your forward momentum, by driving your car into the ground with it's (albeit small) impact.

Quantify the amount of force that several hundred gallons of water might have (over a long drive), and of course, you'll see that it really does add up to some force being exerted on the vehicle. Of course, the difference is negligible, I'm sure.

Obviously you need to push on the dashboard to counteract the rain. :P

I'll drive, you push. :) (You'd be holding the dashboard anyway, wondering why you can only see mud where a road should be.)

I took my SE-R out yesterday during the first rain of the season. I was only able to achieve 37 mpg, running windshield wipers, lights, and defroster fan on my 10 mile country road loop. Conditions were moderately steady (~.5"/hr) rain with 25 mph cross winds, 55 degree F temperature, and a few puddles with 1-2" deep water.

Last week, I drove the same road in the same car and got ~50 mpg on dry roads with 10-15 mph cross winds and temperatures about 10 degres F higher. So yes, the 23% drop in mileage sounds remarkably similar to the mileage drop I experienced.

My coasting distance is reduced anytime I can hear the tires "singing" on the wet road. The singing noise is produced by the tire tread displacing the water on the road when it makes contact with the road surface and then sucking it back into place as it lifts the tread off the road surface. Air is quicker at filling in the low pressure zone formed when the tread is lifted off the road. If you total up the mass of water on the road that is being displaced away from the tire's contact surface as it sets down and then sucked back into place as the tread lifts, it is considerably much more than the mass of water in the form of raindrops that is being displaced as it strikes the vehicle.

For me, a 83mpg commute on dry roads can turn into 71 mpg commute in moderate to heavy rain, average speed 55mph.

wet
 

I've got a table published by the EPA, which I believe has some numbers.I do remember that the SAE recommends coastdown testing be terminated if roads get wet enough to leave a tire track on the road.
I should have posted that table years ago.I'll dig it out,sorry!

10-20%+ depending on the amount of rain.

regards
Mech

I notice nothing in the rain. not even ice storms closing in the grille and making peep holes out of the winshield..but if its foggy, maine version, cold and warm and upside down and rain and ice all at the same time...I think i am down 1% :confused:

it is the engines and their mixes, and designs. Blaming weather for a cars behalf is like trying to live in the arctic with bermuda shorts..it is rather silly.:rolleyes:

my fathers rigs never change, no matter what, and my locale is astounding, it has everything known to mankind, even the freak giant hail. It is where I learned to get angry at most machines that start losing..ya know in the wind for one, like god wasn't ever gonna have any for your journey and ridiculous car. These statistics written, why can't it make changes in machine integrity? there are winners...so very backshelfed.:(

Bgd73:

Yogi Berra could not have said it better.

found my data
 

I dug through my rat's nest,here's what I have:
Two citings,by AARP,and Robert Sikorsky( How to Get More Miles per Gallon),claim up to a 1-mpg loss due to rain.Neither gives a source for the data and both refer the reader to a table from the EPA which shows other weather related mpg data.And no reference baseline mpg is given so it's impossible to come up with a percentage difference.These data were published circa 1975,when cars were averaging about 13-mpg.
The third citing is from my textbook,Internal Combustion Engines and Air Pollution,by Obert,1973.On page 58,Fig.2-28 there is a table constructed from data taken from road tests of a Simca Aronde automobile just before and during a light rain.
The table shows mpg from 20 mph,to 60 mph,both wet and dry,and for the velocity spread,mpg suffers by 9%.
Lower mpg is attributed to increased air density,the retarding effects of tires pumping water( 832 X density of air ),and water vapor ( 100% R.H.) displacing oxygen in the charge,which softens the pressure rise of combustion ( like water injection or ADI ).
The 14% cited in the other source may be attributed to greater frontal area of modern tires,which would have greater pumping losses in the wet,and maybe also higher driving speeds since the demise of the 55-mph National speed limit.
I took a digital photo of the table and Al will post it later today when things slow down here at the copy center.

Table: Wet Road vs MPG
 

Let's see if I can get this image up,

That rain adversely affects MPG (snow is worse) squares with my experience, but I have not found any way to avoid driving in the rain and snow.

Here's an example I use:

I need to run to the store to pick up some items some time this week. If it's raining today, I can look at the weather forecast and see that tomorrow will be dry. I'll do the errand then, instead of now. Besides, it's more pleasant to be out and about in dry weather. :)

Sort of like what I do often, but I'm more like: It'll be dry and warm in two days, so I'll bike to town then...

Although I also have no test data to add, the total mass of water moved by the tyres per mile in heavy rain must be substantial, some of it is thrown up off the road too. That all takes energy

https://cumminsengines.com/uploads/d...el_economy.pdf

This is generally interesting but page 30 (check the index) has a paragraph on rain and mpg

After working in theme park maintenance departments for 20 years, I can state that roller coasters do run faster with a damp track than with a bone dry track.
That's with steel wheels, on a steel track.

Unfortunately, that scenario doesn't help us on the road.
I'm quite sure (without actual data :o) other side effects of temperature, electrical load, puddles, etc, cancel out any slight gain due to reduced frictional losses.

It is the tires pumping water away and sucking it up on the back thread mainly, but also increased air resistance from droplets on the bodywork, AC to prevent fogging, lights where they would not be needed in fair weather.

I reset my Trip B counter at every cold start so I see what economy my commute yields.
My best trip in rainy conditions is about 3.7 l per 100 km.
My best in dry conditions is 3.1 l per 100 km, same route, and I have hundreds of dry commutes all beating my best wet FE.