Californian Study of Rolling Resistance:
http://www.energy.ca.gov/2006publica...0-2006-001.PDF
Page 27:
Quote:
Reductions in tread thickness, volume, and mass are among the
means available to reduce rolling resistance,
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Page 63 - Has some interesting charts regarding vehicle energy consumption
Page 64:
Quote:
a 10 percent
reduction in rolling resistance will lead to a 1 to 2 percent increase
in fuel economy and a proportional reduction in fuel consumption.
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Page 67:
Quote:
Reducing a tire’s aspect ratio—
that is, its section height relative to its section width—should reduce
hysteresis if it is accomplished by shortening and stiffening of the sidewalls.
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Page 68:
Quote:
These experiments suggested
that rolling resistance declined by 26 percent over the entire wear life.
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I could not find anything in the article regarding the "hardness" of a tire, but it stands to reason that the harder the material the less it flex and thus will experience less hysteresis. The above example of a steel train wheel is a great example of an extremely low rolling resistance wheel.
The article does discuss to a large extent the relationship of the performance of the tire compared to the efficiency (rolling resistance) of the tire. It generally states that their is probably somewhat of an inverse relationship but that tires could be designed to improve their RR without adversely effecting its other properties.