What factor causes the most difference between the selected course and the magnetic heading while tracking a VOR radial?

When tracking a VOR (VHF Omnidirectional Range) radial, the primary factor affecting the difference between the selected course and the magnetic heading is wind. As an aircraft flies through the atmosphere, it is inevitably influenced by wind, which can come from various directions and at different speeds.

This wind drift can cause the aircraft to deviate from its intended course. For instance, if there is a crosswind, the aircraft will drift off the radial it is supposed to follow. Pilots must constantly monitor wind conditions and adjust their heading accordingly to maintain the desired track over the ground. By compensating for the wind, they can align the aircraft's heading to ensure it stays on the correct path toward the VOR.

Other factors like aircraft weight, altitude changes, and flight speed can have effects on performance and handling characteristics of the aircraft but do not directly impact the relationship between the intended course and magnetic heading as profoundly as wind does. Wind is the dominant force that alters the ground track relative to the intended route defined by the VOR radial.