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 Hopefully new data comes out, as some tire models are outdated.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.