Increasing from 26 to 44 psi increases mpg from 31 to 32 on a Ford Focus.
See figure 3 on page 17:
http://www.energy.ca.gov/reports/2003-01-31_600-03-001CRVOL2.PDF
Low rolling resistance tires help significantly but unfortunately the article doesn't identify tire brands tested.
Also note the chart showing that worn tires have lower rolling resistence which improves mpg significantly.
Also reduced wet traction at low pressure
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=3
See figure 3 on page 17:
http://www.energy.ca.gov/reports/2003-01-31_600-03-001CRVOL2.PDF
Low rolling resistance tires help significantly but unfortunately the article doesn't identify tire brands tested.
Also note the chart showing that worn tires have lower rolling resistence which improves mpg significantly.
Also reduced wet traction at low pressure
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=3