your garmin gives speed across the ground, and actually only in a horizontal direction. So it lies when you are on anything other than the flat. The degree of lying depends on the slope.
In fluid dynamics terms this is a pretty complex set of euqations (I might have had a stab at it 20 years ago), mostly because you as a human bean is not uniform, and then attach you to a BSO which is more non-uniform and is varying in its non uniformity. You could make several approximations to uniformity, but effectively the question you are asking is how much more drag is being created by me cycling into a wind, assumed headwind, than cycling into still air.
So effectively normal drag = 15mph (say) into still air, effective windspeed 15mph
New drag = 15mph "speed" plus 15mph wind = 30mph "drag"
I believe form drag scales with square of speed, so x 2 speed = x 4 energy output required to overcome that aspect of drag, then you have friction losses (drivetrain plus skinfriction), then you have your biomechanical efficiency, then your biochemical efficiency
I'd say go find a wind tunnel, or a CFD expert.