Dry roads vs wet roads

Any type of moisture on the roads is a killer.
Cold is a killer that can be mitigated to some extent.
Kansas winds can cost me as much as either of the above.
Etc.

I have not isolated wet road impact as clearly as you, but would guess extreme cold (like 0F) has a greater impact than a wet road, unless the surface is more than good and damp.

Now rain brings more wind resistance into play as well as the rolling resistance penalty. How wet were your roads and was it raining, also?

 

Yes your car is doing a lot of work to throw up all that spray behind you, that is work done by gasoline. I suspect you slip tiny amounts in the wet, and are less efficient. Cold air with rain in it is thicker than warm air without rain. You are running wiper motors, that power comes from gasoline.

 

Not to be too picky, but moisture aside, cold air is more dense than warm air....

 

Technically you're correct, but I think air density isn't too big a factor, otherwise the temperature alone would explain this. I too have noticed that driving in the rain reduces mpg, but I'm thinking it's more about the road surface. Flinging the water around, maybe the road is a little stickier? Not sure, but I remember one long road trip where it was raining the whole way, I expected 50-52 mpg, but got maybe 44. My wife kept the heater on, but that wouldn't explain the whole effect, especially once the cabin was warm. It's not the only case, but the most pronounced one I remember.

 

On long trips, cold doesn't have much of an effect on MPG like it does on short daily trips. Once the car the car is warmed up you're past most of the negativity of cold temps.

Wet road will always get you no matter the temp. The wetter the road the worse it is. A lot of energy is required to pump water off the road, through your tire treads and into the air or otherwise out of the path of your tire. That energy ultimately comes from the fuel in your tank. I've never quite figured out wet lightly wetted roads reduce efficiency but it does anyway. lol My only guess is that instead of having to pump water out of the way you are picking up and moving a smaller amount of water. Anytime you move something you are doing work and that requires energy.

 

From my expeience it doesn't seem to make that much of a difference

 

I noticed the same thing very clearly yesterday. We drove to church Sunday morning. It is a 17-mile trip. We usually get 58-60 mpg per the onboard computer. This time, the roads were still wet from a rain earlier in the morning. I got only 52 or 53 mpg.

 

Never gave it a lot of thought.

Cold air = lower MPG because the denser air makes you burn more gas...however you get more HP because there's more oxygen/fuel in the cylinders. This should be so regardless of fuel formulation for the season.

I never thought rain really impacted MPG all that much. Of course, I don't have many days I'm driving in a hard enough downpour to consider the roads "wet" in a way that adds considerable resistance to a rolling tire. I'd be more inclined to blame lower air temperatures (as noted above) for the loss of MPG.

 

The water splashed by the tires and picked up by the body causes a lot of drag. Next time, look at the wheel wells of cars you're passing and the spray will show how much air is picked up and tossed out the side with the spray.

Water is just under 800 times denser than air so a little bit of the liquid stuff has a big impact.

Bob Wilson

 

Any car loose MPG in rain. 1st rolling resistance goes up, 2nd all the water falling on the car and being accelerated to 60mph.

 

You get more horsepower at wide open throttle with colder air. You're still limiting the horsepower with the throttle, which restricts the amount of air going into the engine. If the air is a little denser, very slightly less opening of the throttle is required to achieve the same amount of power output. However this is more than offset by the increased wind resistance of having to push the car through the denser air.

Humidity also lowers the density of air. The drier the air, the more dense it is. Elevation has a huge effect on air density.

The most efficient driving condition would have the vehicle at a higher elevation, with high humidity, and warm temperatures.

All that said, I haven't noticed a difference in MPG when it rains, but then again I drive to the side of the worn grooves in the road. You can feel the massive deceleration when you drop into the grooves and hydroplane a little. I notice this when I'm changing lanes.