There is really only one force acting on the car: A centripetal force mv^2/r, which is pointing towards the center of the turn. This centripetal force is due to the effect of friction.
In other words, as the car goes around the turn, there is an effective centripetal force pointing towards the inside of the turn, and this force is due to the friction between the car's tires and the road.
Since these forces are one and the same, you'll want to set the equation for the car's centripetal force and frictional forces equal:
M V^2 / R = mu[k] * N
Normal force = M*g
yielding
M * V^2 / R = mu[k] * M * g
Doing this allows us to solve for mu[k].
mu[k] = V^2 * [ 1 / (g*r) ]
g is a constant on the planet earth, as is r (the car is still going around the same curve). Thus mu[k] is being related to the velocity.
if the frictional force is decreased by 1/3, then the velocity must be adjusted accordingly, by a factor of sqrt (1/3).
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