Originally Posted by RxZ
Tires are made out of different compounds, roads are made of different materials. Each has their own coefficient of friction differing from dry to wet. To have a rule of thumb, you would have to know all the variables as mentioned.
The only rule of thumb is, whatever traction you have when dry, there is less when wet.
You might want to think that you actually, on a clean wet road, assuming decent tires and a good rider, there is more traction there than you think. You really just have to be smoother and a bit slower in the rain. You don't do nothing sudden in the rain, be it steering inputs, braking or acceleration. I have seen good road racers only loose a second or so per lap on a 1.5 mile course in the rain. Slicks are another story.