Rain.
 

Despite Scotland being a rather wet part of the world I have not driven a great deal in the rain since trying to improve my fuel efficiency. I have the past couple of days. What I have noticed is just how much wet roads impact on the distance I can coast. I am not talking about standing water here, but just roads wet after a spot of rain. I reach my coasting point at the usual speed, slip into neutral - and fall short of my intended target. And sometimes by quite a bit.
This retardation obviously impacts all of my driving, even when under power, but until I tried coasting I was not aware of what a huge difference it made.
Another aspect of FE it impacts is cornering speed. For junctions around town, where I can normally turn on two wheels and a door handle, I have to slow quite a bit more, so that too impacts on FE.
I'll just have to suck it up. Winter approaches, and if there is one thing you can say about a Scottish winter is it will be cold and wet!

City traffic is indeed already bad for mileage, but at least the colder weather and the increase in air humidity make it safer to run leaner and overcome the NOx emissions issue usually associated to lean burn.

I tend to drive outside of the grooves in the road to improve FE when it rains.

As far as cornering speed; I don't tend to slow down any. Then again, I've spun out twice on a corner well known to me due to not slowing down in the rain.

I'd estimate a car to have 75% traction available in the rain, of which, most people probably use 20%, even in dry weather. People hate to feel lateral Gs of any amount.

The roads around here aren't really grooved. We don't have that much in the way of heavy vehicles. Some of the trunk roads can be bad though.
As you say, most people never get near the limit of adhesion on corners. If the tyres are not squealing you are no where near the limit. I did a racing driver's course at Knockhill Race Track, near here, and it is quite something the speed you can get through corners. Even on the bog standard SEAT Leon Cupra they start you off on.
What I find with my Jazz is that, in the wet, it starts to understeer very quickly. And that is without elevated tyre pressures. I think the Firestone Multihawk 2 tyres, on the front, are just rubbish. Tons of tread but no wet grip.
I love pushing a car on a bend. There is a part on the nearby M90 that turns through 270° and there is always someone who wants to overtake as you turn into it. They then have to do about 20 mph more than I do just to get past. And I just nail it round there. Unless it is a four wheel drive Audi or the like, they have no chance.
http://i.imgur.com/8m3bC9V.jpg

And me at Knockhill.

http://i.imgur.com/k40FsIq.jpg

Remember the "resistance" you felt when wading thru water? That's what the tyres feel, too, only instead of pushing the water away, the tyre is squishing it away thru the sipes on the tyre...parting water from between the blocks & sipes to the sides of the tyres takes energy, which is resistance.

But, ironically, a damp day has less air-density for the car to penetrate, so air-resistance should be somewhat reduced. The worst time for speed-trials is on a COLD, DRY day, when air-density is highest.

Not if you drive a hybrid ... or drive LIKE you have a hybrid.

My highway mileage (at very sedate speeds, I will add) almost always pulls my average down.

And don't forget the non-trivial amount of energy required to pick up some of that water and "throw" it away from the tires.

I recall reading a calculation someone made on the volume (mass) involved, and it was surprisingly large.

Solution! Always drive on slicks!

I'm sure it's an oversimplification, but I estimate I see maybe a 10mpg loss when the roads are damp, and might see 20-30mpg lost if the water is visible. Add to that, I sometimes have to run the A/C compressor to keep the windows from fogging and a 95mpg highway trip can easily turn into a 60mpg trip.

Interesting numbers - emphasized by the Insight's sensitive & small "lean burn window". Increase resistance enough to close that window and... OOF!

I don't drive enough to have collected any "rain vs. dry" numbers for a conventional engine on the same route.

This certainly tracks with what I see.