Yes air does return more energy than rubber and steel. However the less it is compressed or disturbed the less energy lost. The same holds true for rubber and steel, the less it is flexed the less the energy that is lost.
hard rubber looses less energy.
http://www.vittoria.com/index2.php?o...=358&Itemid=71
From the box on the second page.
"Rubber compounds affect rolling resistance because they absorb different amounts of energy. In rough terms, thick, soft rubber usually absorbs more energy than thin, hard rubber. "
Caster Wheels - Performance TPR Wheels - Soft Rubber Wheels - Hard Rubber Wheels - Mold-On Rubber Wheels - Elastomeric High Tensile Rubber Wheels
From the comparison of types of caster wheels. Read the descriptions of the Soft rubber wheels and the Hard rubber wheels.
"Soft Rubber Wheels are a good choice for those applications where the average load is light with occasional loads using the full capacity of the wheel. Soft Rubber Wheels are manufactured of quality materials with a resilient rubber tread bonded to the hard rubber core."
"Hard Rubber Wheels are used where low cost, easy rolling wheels are required. Hard Rubber Wheels offer good floor protection, are quieter than steel wheels, and are strong and chip resistant."
Occasionally someone makes an advance in material science like adding silica to tire rubber allowing softer rubber to have a lower rolling resistance and better wearing characteristics. This is the exception and not the rule. It can be added to harder rubbers too but they won't grip as well.
My father designed tires. I learned a lot from him.