I don't know what your circuit is, but the symptoms you're giving make me suspect earthing issues. Perhaps you could post the circuit?
I'm unclear now whether the problem is that the data is getting garbled during transmission, or is corrupted in the first place. Perhaps a test sketch which produces dummy data would tell you which was the case?
How is the Arduino powered in the car? It'll be very vulnerable to any electrical noise on the power supply or any of its inputs, so you'd need to buffer them to ensure reliable operation.