Two years ago congress funded a tire study on rolling resistance. The study is a bit boring, but extremely informative.
Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance -- Special Report 286
Regarding Tire pressure:
"For typical passenger tires inflated to pressures of 24 to 36 pounds per square inch (psi), each 1-psi drop in inflation pressure will increase rolling resistance by about 1.4 percent. Hence, a drop in pressure from 32 to 24 psi—a significant degree of underinflation that would not be apparent by casually viewing the shape of the tire—increases a tire’s rolling resistance by more than 10 percent. At pressures below 24 psi, rolling resistance increases even more rapidly with declining inflation pressure. "
This indicates a significant change in efficiency by maintaining high tire pressure.
Other relevant things of interest:
-It will soon be easy to find a reliable rating for RRC (rolling resistance coefficient) on tires
-A tire's RRC improves with wear, due to loss of tread. The tread is responsible for significant tire deformaties that generate heat (read: energy loss)
-According to a survey of approx. 150 tires by 3 major manufactures, 16" wheels tend to have the best overall average RRC (on a side note, the lowest rating was .0065ish from what I think was only a 14" tire, maybe 15")
-RRC range of the study was .0065 to .0150, with average around .0100
-The study found that OE tires generally had significantly lower RRC values, likely due to manufactures trying to meet EPA CAFE (corporate average fuel economy) regulations
-Only 13% of the spent fuel energy reaches the wheelsin urban driving, of this: 3% goes to overcome aerodynamic drag, 7% to overcome rolling resistance, and the rest goes to breaking at 6%(I presume this really accounting for acceleration, but I'm not sure). Highway driving 20% of energy reaches the wheel with this break down, 11%, 7%, 2%. Notice how much more effiencient the system is on the highway with regards of energy reaching the wheel. Also of interest is the obvious increase in air resistance loss (also important to note, that all though air resistance went from 7 to 11%, the energy increase being used for over coming air resistance is more then 4% due to an increase in spent fuel).
This link shows more information for the above:
Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance -- Special Report 286
The two links posted reference different pages of the same report.