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

[PaleMelanesian]

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)

[cfg83]

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

[Christ]

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.

[Matt Herring]

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.

[moorecomp]

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.

[RobertSmalls]

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.

[Christ]

Originally Posted by RobertSmalls

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!

[MetroMPG]

Originally Posted by moorecomp

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.

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.

[Piwoslaw]

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.

[Christ]

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.