I But one day, a day my wife joined me, there was a strong headwind and rain and I average 285 W/Mile a 20% increase. With the cruise control I can keep the speed constant and there is little traffic along the route so I am convinced it is the weather that made such a difference.
You make an interesting point here !
Above about 60mph, the dominant energy drain on the car will be due to aerodynamic drag. Rolling resistance is usually a constant irrelevant of road speed. A small percentage change in the weight of entire vehicle will have little effect on the energy required to push the car along at a steady speed.
However, you mentioned that there was a strong head wind and rain. A 20 mph headwind will be like driving 15-20mph faster and will increase energy drain.
Rain on the road will effect rolling resistance and reduce the friction/grip of the tyres on the road, BUT there will be a significant amount of energy needed to constantly displace the water on the road as well as all the water going around the treads and being flung off behind the tyre and up into the inner wheel arch.
If there is half an inch of water on the road each tyre will be constantly displacing that weight of water multiplied by the contact patch and vehicle speed. This adds up to considerable amount of energy. I don’t have the equations to hand, but from what I remember seeing in studies on this, rain on the road can increase energy usage by 15 to 20% or more, dependant on speed.
So loosely speaking, head wind and rain combined could dramatically increase your energy drain by 30-40%
I've also seen debates on other weather aspects, such as the marginal effect of absorbing all the energy of large rain droplets hitting the car at speed multiplied by its frontal area, and so on.