The official terminology is usually rolling resistance coefficient, or RRC. For automobiles, the RRC for a 'green' (energy efficient) tire is about 0.01, compared to a steel wheel on a rail, which has an RRC of about 0.001. This is why railroads are the most efficient mode for long-haul freight. See table in http://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.htmlYea, rolling resistance is probably better terminology, because that's really what we're talking about. But my point was that rolling resistance is a (relatively) minor effect, and you are always going to have some of that no matter what wheels you use.