They found that inner tubes averaged 20.3 miles per hour—around 2 mph faster than the plastic sleds.
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Sledders on inner tubes went faster and generated more kinetic energy than those on plastic/hard foam sleds; mean 20.3 mph (95% CI 19.5-21.0) versus 18.3 mph (95% CI 17.8-18.7), p < 0.0001 and mean 2,136 J (95% CI 1,752-2,520) versus 1,707 J (95% CI 1,478-1,936), p < 0.05, respectively.
In addition, there is air resistance to consider - the air resistance is dependent on the shape of the object, not its mass, so two objects with identical shape but different mass will have the same aerodynamic drag, allowing the heavier one to go faster.