Does anyone know the coefficient of friction for an A rated tire?

I can’t give you an exact number on that, but I the “A” traction grade is the second-highest rating given to a tire, AA being the highest. The traction grade applies to wet traction, and is given as

A – Above 0.47 G for asphalt and 0.35 G for concrete. This means the tire should allow for G forces at or above those listed without breaking loose on wet pavement.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=48

This does not tell the whole story. The amount of traction you’ll get with any tire will vary with vehicle weight, inflation pressure, and tire wear. It also applies to straight-line traction, not lateral traction, as you’d see on a skidpad test. There, you have sidewall construction and vehicle characteristics to consider as well.