Unity 5 wheel collider throttle "stuck".

Whenever I release the forward key for my car, the car continues accelerating for several seconds before it finally begins to decelerate. I am using mathf.clamp(input.getaxis(“Vertical”),-1,1) so is this causing the issue? Also, the car is accelerating slowly, and changing both the torque and the drag of the rigid body, it cannot accelerate from 0 to 60mph in less than 8 seconds. The speed value is rigidbody.velocity.magnitude * 2.237. The 2.237 is to convert it from meters per second to miles per hour. The rigid body’s mass is 1500. How can I fix this?

Finally fixed the problem, the forward friction wasn’t great enough so the wheels were in a permanent skid, even if they were not accelerating. Increasing the wheelcollider forward friction fixed both of my problems.