I can't link rotation to both X position and Y position of another layer

iExpressions is the easy way. Here's a tutorial:

The key to getting the rotation correct is calculating the circumference of the wheel. That's just Math.PI * width.  

I'm leaving for a fishing trip right now, but if someone else has the time, they can give you the length of a line at an angle. I don't want to give you an incorrect expression, but it's just the length of the hypotenuse. Just use that to determine the rotation multiplier.

Or, spend a few bucks and just buy iExpressions.

Perfectly and easily possible. You simply multiply the rotation amount with a cosine and sine based on the inclination angle, respectively. You would just have to remap them through linear() functions to get whatever value ranges you need and you may need a safeguard for when the sinus/ cosinus values turn to zero at specific 90 degree angles. Something like this perhaps:

mCar=thisComp.layer("Car");
mPos=mCar.transform.position;
mRot=degreesToRadians(mCar.transform.rotation);

//angular multipliers

mX=linear(Math.cos(mRot),0,Math.PI,0,1);
mY=linear(Math.sin(mRot),Math.PI,0,1,0);

//fictional circumference of the wheel

mCirc=mPos%500;

mCirc*mX*mY;

This isn't exact, but should give you the basics. It could be fancied up in a million ways like calculating the actual circumference/ distance travelled based on the wheel diameter and of course tweaking the rotation ranges.

Mylenium