We have noticed in the past when drivers show up wanting us to fix poor mpg's that they were runing on cheap or worn tires that slipped a great deal; in rain the slipping isn't noticeable. The cure was a new of better set of tire(s) especially when one side is more worn than the other. Also you might check tire pressures about 35 psig, not much higher or lower. Just out of curiosity what are your tires? And might the diffrence between dry and wet driving you brake a lot more in the wet? cheers.
