1.Rain.
At the family holiday feast preparations, the kid asked his mom why she cut the ends off the ham then tucked them in the pan before baking. She replied that that is what she learned form watching her mom, and to ask grandma. Grandma's reply was the same as the daughter so she turned and asked the Matriarch of the family, the Great Grandmother who replied; small pan.

People still drive in the rain with the windows open.
Great Grandpa didn't have Air Conditioning that automatically ran with the defroster to remove the window-fogging humidity.......

Water on roads dramatically reduces free rolling. Ever hit a puddle at speed?
Better MPG might be because we drive slower?

2. Using the A/C.
I missed the part about the reduction in MPG caused by the drag of having open windows...even a little bit.

I didn't see the part where the ScanGauge was compared to actual arithmetic involving real miles and real gallons.


But, silly me...
I got rid of my Dakota because the A/C broke and it cost more than the truck was worth to fix it.
To me, the conversation is academic. I chose comfort.

Me too. Besides, it evens out in the end; I'm a more patient driver and more able to save gas when I'm more comfortable.

I totally agree. If I'm not comfortable there is no way I'm going to hypermile. I'm going to get to my destination as quick as possible.

-Jay

{Bottom line is the more efficient cars become the much greater the cost of running the AC. It's more exponential than linear and if we ever build mega mileage cars we will also have to address the gross inefficiency of conventional AC systems, even with the best modern designs of compressor.}

I believe this only applies to traditional belt driven ACs. The Prius uses a variable speed electric compressor. It doesn't directly suck off energy going from the engine to the wheels. There will be a mpg penalty of some type, since the energy it takes from the battery has to be replenished in someway, but then the battery is cooled by the cabin air. So AC use may keep it at a more efficient point.

I've heard BMW was experimenting with regenerative braking on non-hybrid cars to run accessories. With the regen brakes and a beefy battery, it may be possible to replace all the belt driven add-ons with electric.

Driving in rain helps mileage a little sometimes - a little water on the road makes it more slick and lubricates the tread reducting traction and lowering rolling resistance a little and raising the MPG - however if it gets too deep then it can slow you down. It also adds moisture to the combustion which helps generate more power in the engine. Using a little heat for the defroster keeps the windshield clear but since the cars have gone to a single stupid control for the vents you can't get warm air on the windshield and cold air on you like my old VW Rabbit with the separate fresh air vents that only put out cold air and separate control levers for feet and windshield. Ahhh those were the simpler days when a simple lever turned something on and off as much or as little as you wanted. Now you have dash - dash and feet - feet - feet and windshield with defrost - windshield with defrost.

Adding moisture to the air/fuel mix might help.

Rolling resistance, AFAIK, is not caused by tread friction on the road (and if it was, tires would never last 40,000 miles). It's caused by having to deform the sidewall at the bottom.

I sure would like separate controls for things.

i'm thinking light rain and/or roads w/ good drainage keeping standing water to little or nothing.

oh, and we call slick roads at the beginning of rainfall "black ice" in FL.

i can vouch for that, i take a hit of about 4-6mpg in my civic running the a/c. my explorer is almost un noticeable.

The differences are insignificant in my cars. The Matrix is small but I think the electric compressor used in it instead of a belt driven one helps. However, its cooling effect is not exceedingly strong at the same time.

Light rain may improve mileage since it cools down the road surface turning it less sticky than in a hot day. As you all know, tarmac may start melting if the temperature is high enough.

Heavy rain will decrease mileage even if the tarmac has cooled down, for the reasons mentioned.