We did a couple of day stay ~100 miles away at the beach last weekend. While calculating the mpg's of the overly thirsty impreza impreza after it a fillup it dawned on me that both sides of the trip were were in about 80% rain. So any idea how much rain changes mpg?
I just had this conversation on the weekend with my buddy who's fanatical about his fuel economy. He told me exactly what the difference was, but I can't remember. My guess is, that it was probably 10-15% worse.
Depends how much rain, how rutted/puddled the road is, if you drive in said ruts/puddles, and how good your tires dig in the water. Somewhere between negligible and somewhat noticeable.
Often times with rain you get wind, which I would think would be a bigger factor
I don't think it's the rain per se, pushing water away from the tires; but the inconsistant and fluctuating speed of other drivers. Speed up, slow down, oh oh, there's a truck lets pace it for a while. 
It's going to vary depending on the vehicle, and how many tricks the mfg uses to get higher economy in addition to factors already mentioned.
-Most HVAC systems use the A/C to dry the air when using defrost/defogging settings
When doing my old commute (65 miles each way almost all highway) I would see a 15-20% drop in mpg on my 2011 Elantra if there was rain.
Depends on how much rain is falling, but in my POS Dakota, maybe 5% on a "dry" 21 mpg.....
My knee jerk response would have been that your mpg would be greater in the rain since you would tend to drive slower in most circumstances but I can see the "physics" on why with greater rolling resistance, the mpg would be less.
I also think some cars would respond differently based on the additional humidity being pulled into the combustion chamber with the air.
Never really thought about it.
Tires would make a big difference. If you are wearing DOT street legal autocross tires, the lack of tread means it is pushing a lot of water in front of the tire, instead of channeling it through the tread blocks. It would also mean you are hydro-planing more (less traction to the road).
I doubt it is very linear- It is highly likely it would vary significantly from 1 car to another...
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