Renewable Fuel Standard Improvement Act - Amends the Clean Air Act to: (1) include the Secretaries of Agriculture and Energy in renewable fuel program activities under such Act; (2) require the Secretary of Energy to periodically update the baseline lifecycle greenhouse gas emissions; (3) revise the definition of "biomass-based diesel" to include certain renewable fuel produced after December 19, 2007; (4) revise the definition of "lifecycle greenhouse gas emissions" to base the measurement of such emissions on recent observable data and consideration of regional differences of renewable fuel production, rather than on indirect land use changes; (5) expand the definition of "renewable biomass"; and (6) require a periodic review of adjustments in greenhouse gas reduction levels.