InverseTransformDirection - Odd behaviour on x axis

Hello everyone,

I'm currently trying to read pseudo lateral G Forces of a vehicle. I know that using InverseTransformDirection of the velocity vector I can get the position of the body relative to self:

Code (CSharp):

1.         lateralVelocity = transform.InverseTransformDirection(rigidbody.velocity).x;
2.  

My understanding of the x axis is that left is negative, and right is positive.

The Problem

At low speeds the velocity reads correctly: left is negative, right is positive. However once the vehicle picks up speed, when turning sharply it will read in the opposite direction!

Has anyone come across this before? I feel like there's probably something daft that I'm missing.

Thanks for reading,
Ismael