[Intrigued]

One thing that can help mileage when it's raining is that the water vapor in the air helps to cool/dense up the air going into the engine, which can then bump up timing, etc. The guys who were selling those quack "mpg improvement kits" that were basically a water jug with a vacuum line running to it were trying to use that idea, but were introducing the vapor in the wrong place and time.

Back before the computer days a person could easily advance their timing by 4 to 6 degrees on cool, wet nights, and not notice any spark knock, all the while getting much better gas mileage!

...and as anyone would presume: your mileage may vary!!!