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.
What about same bike, same tires, same everything, on the same road, only difference is the road is wet or dry. Could you assume 20% less grip? 50%?