It's quite possible for the interest rate to be reduced in the course of a Chapter 13 bankruptcy.
But if you buy a car you can't afford and trot into bankruptcy court asking the trustee to both cut your interest rate and give you an expanded allowance for the vehicle because you can't afford to operate it, I think you're shooting yourself in the foot on that latter request. And if you're stacking up cars like cord wood, saying "But my kid drives our third car" isn't likely to get you anywhere when the trustee asks why you're not getting rid of the car you can't afford.

