Higher altitude the specific range (V/FF) increases, best range speed increases (tangent to curve), min total drag, ie best endurance also increases.
Headwind move origin of tangent to curve to the right, tailwind move the origin to the tangent to the curve left.
Therefore in nil wind , fly or hold as high as possible in a jet. When flying east - stay high with jetstreams, flying west stay low out of jet stream. North/south, generally high, may be have reduced effective TAS due strong crosswinds from jetstreams (can be 100-150kts+).