(Undated)  --  Portland is ranked among the ten "greenest" cities in the nation, according to a new WalletHub.com study.  The analysis shows Portland is ranked eighth, with San Francisco placing at the top of the list.  The report ranks Portland 20th in greenhouse gas emissions per capita, 16th in the percentage of green spaces; 13th in percentage of commuters who drive, and fourth in bike commuting.  Seattle is eleventh overall in the WalletHub rankings.