A bicycle's performance is extraordinarily efficient. In terms of the amount of energy a person must expend to travel a given distance, cycling is calculated to be the most efficient self-powered means of transportation. In terms of the ratio of cargo weight a bicycle can carry to total weight, it is also the most efficient means of cargo transportation. Mechanical efficiency From a mechanical viewpoint, up to 99% of the energy delivered by the rider into the pedals is transmitted to the wheels (clean, lubricated new chain at 400 W), although the use of gearing mechanisms reduces this by 1–7% (clean, well-lubricated derailleurs), 4–12% (chain with 3-speed hubs), or 10–20% (shaft drive with 3-speed hubs). The higher efficiencies in each range are achieved at higher power levels and in direct drive (hub gears) or with large driven cogs (derailleurs). Energy efficiency A human traveling on a bicycle at , using only the power required to walk, is the most energy-efficient means of human transport generally available. Air drag, which increases with the square of speed, requires increasingly higher power outputs relative to speed. A bicycle in which the rider lies in a supine position is referred to as a recumbent bicycle or, if covered in an aerodynamic fairing to achieve very low air drag, as a velomobile. On firm, flat ground, a person requires about 60 watts to walk at . That same person on a bicycle, on the same ground, with the same power output, can travel at using an ordinary bicycle, so in these conditions the energy expenditure of cycling is one-third that of walking the same distance. Uphill and downhill speeds vary according to the slope of the incline and the effort of the rider. Uphill cycling requires more power to overcome gravity and speeds are therefore lower and heartrate is higher than during flat riding conditions. With medium effort a cyclist can pedal 8-10 km/h up a gentle incline. Riding on grass, sand, mud, or snow will also slow a rider down. Without pedaling downhill a bicycle rider can easily reach speeds of 20-40 km/h down a gentle 5% slope and speeds exceeding 50 km/h on steeper inclines. Energy output Active humans can produce between 1.5 Watts per kilogram of body mass (untrained), 3.0 W/kg (fit), and 6.6 W/kg (top-class male athletes). 5 W/kg is about the level reachable by the highest tier of male amateurs