Finding the coefficient of friction

[return42]

Homework Statement

A race-car driver discovers that he can accelerate at 4.0 m/s² without spinning his tires, but if he tries to accelerate more rapidly, he always "burns rubber." What is the coefficient of friction between his tires and the road?

Answer behind the book:

Spoiler

0.62

Homework Equations

Kinetic friction force = u_kN
Static friction force <= u_sN

The Attempt at a Solution

I'm really lost with this one. I tried starting with Newton's second law and end up at:
u_kmg = ma
u_kg = a
u_k = a/g = 0.41

Which is not the answer behind the book. Is this considered static friction? I'd think not because the tires are slipping relative to the ground. Anyone can put me on the right path?

 

[PhanthomJay]

It is static friction up to the point where the tires start to slip. So I get u_s = 0.41. I don't understand the given answer, unless some info is missing.