You may need more pressure yet? I run 9.75 psi in a 15 x 33 at 3000 lbs. Sounds like 17 or more might work? The clue was the fact that the car moved around at 13 psi. At 15 you are barely stable then, not enough to make them work right . I bet you will see a 1.30 or better on the next outing.