I found the solution myself after a (bad) night's sleep on it.

Working Code

void Update () { // NOTE: I am really bad at this (vectors, cross products, dot products etc), hence my comments will be to the best of my understanding. // Time that debug lines and rays will be displayed for. var duration = Time.deltaTime; // Draws a line through the south/north poles of the sphwere. Debug.DrawLine(Vector3.down * 100, Vector3.up * 100, Color.gray, duration, false); // Gravity that affects the vehicle var gravity = Vector3.Normalize(planet.transform.position - transform.position) * 9.81F; // Cross products of gravity to describe planes. var gravityEast = Vector3.Cross(gravity.normalized, Vector3.down).normalized; var gravityNorth = Vector3.Cross(gravityEast.normalized, -gravity.normalized).normalized; // Draw vehicle forward, right and up vectors. Makes it easier to see how it's currently rotated. Debug.DrawRay(transform.position, transform.forward * 0.5F, Color.cyan, duration, false); Debug.DrawRay(transform.position, transform.right * 0.5F, Color.cyan, duration, false); Debug.DrawRay(transform.position, transform.up * 0.5F, Color.cyan, duration, false); // Draw gravity vector and it's plane vectors. Debug.DrawRay(transform.position, gravity.normalized, new Color(0.0F, 0.0F, 0.5F), duration, false); Debug.DrawRay(transform.position, gravityEast, new Color(0.5F, 0.5F, 0.0F), duration, false); Debug.DrawRay(transform.position, gravityNorth, new Color(0.5F, 0.0F, 0.0F), duration, false); // Debug text to verify the absolute values of the gravity and it's plane vectors. text.text = "gravity: " + gravity + "\n"; text.text += "gravityEast: " + gravityEast + "\n"; text.text += "gravityNorth: " + gravityNorth + "\n\n"; // Define "aligned" (plane) vectors that rotate with the vehicle. // For example, gravityAlignedRight will stay pointing along the height plane over the // surface of the sphere and rotate with the vehicle always pointing to the right. // This is used to determine angular difference between vehicles right vector and gravity's aligned right vector. // The reason we need that is because Vector3.Angle() would measure the whole angle between a fixed vector such as // gravityEast and the vehicle right vector otherwise. // gravityAlignedForward is used for yaw and pitch. // gravityAlignedDown is used for pitch and is needed to get the correct dot product for pitch. var gravityAlignedRight = Vector3.Cross(gravity.normalized, -transform.forward).normalized; var gravityAlignedForward = Vector3.Cross(gravity.normalized, gravityAlignedRight).normalized; var gravityAlignedDown = Vector3.Cross(gravityAlignedRight, transform.forward).normalized; // Draw aligned gravity vectors for ease of debugging. Debug.DrawRay(transform.position, gravityAlignedForward * 0.25F, Color.green, duration, false); Debug.DrawRay(transform.position, gravityAlignedRight * 0.25F, Color.magenta, duration, false); Debug.DrawRay(transform.position, gravityAlignedDown * 0.25F, Color.red, duration, false); // And draw the absolute values which helped me set them up. // That is, i had to experiment my way forward to each of these... text.text += "gravityAlignedRight: " + gravityAlignedRight + "\n"; text.text += "gravityAlignedForward: " + gravityAlignedForward + "\n"; text.text += "gravityAlignedDown: " + gravityAlignedDown + "\n\n"; // Get dot products for yaw, pitch and roll so we can set the appropriate sign on the final YPR readings. var yawDot = Vector3.Dot(gravityAlignedForward, gravityEast); var pitchDot = -Vector3.Dot(gravityAlignedDown, gravityAlignedForward); var rollDot = -Vector3.Dot(transform.up, gravityAlignedRight); // Measure angle between the aligned gravity vectors and the vehicles forward and right vectors. // yaw is special in that it uses the aligned forward gravity vector. Don't ask me why, i wouldn't know... ;) var yaw = Vector3.Angle(gravityAlignedForward, gravityNorth) * Mathf.Sign(yawDot); var pitch = Vector3.Angle(transform.forward, gravityAlignedForward) * Mathf.Sign(pitchDot); var roll = Vector3.Angle(transform.right, gravityAlignedRight) * Mathf.Sign(rollDot); // Debug text showing the results of all this... It works, finally. text.text += "yawDot: " + yawDot + "\n"; text.text += "pitchDot: " + pitchDot + "\n"; text.text += "rollDot: " + rollDot + "\n\n"; text.text += "yaw: " + yaw + "\n"; text.text += "pitch: " + pitch + "\n"; text.text += "roll: " + roll + "\n"; } 

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Debug shots

Some shots where everything just works... I am happy now because i can actually port this to the other project! (I hope)

Ok, so i made a mistake when taking the screenshot, pitch from 22.5 to 135 in unity

And with all that said, thanks to everyone that read and tried to help me out. I never want to have to figure this out again...

That being said, i still would very much appreciate input on what i have done and if it can be simplified in any way as well as perhaps some eureka hints you could send my way to actually make me understand what i have done here.

Cheers!