The FAIR Fleets Act amends federal postal law to require the U.S. Postal Service to conduct an ongoing assessment of its vehicle fleet distribution and implement a strategic plan to increase vehicle availability in underserved areas, including rural regions, high-poverty neighborhoods, and tribal lands. The bill mandates that the Postal Service modernize its fleet with fuel-efficient and environmentally friendly vehicles and submit annual reports to Congress detailing fleet distribution by state and region, steps taken to improve service in underserved areas, and recommendations for further improvements. The law defines 'underserved areas' to include rural territories, regions where at least 50 percent of vehicles exceed their service life, counties with mail delivery delays exceeding the national average by 20 percent or more, high-poverty urban neighborhoods, and indigenous and tribal lands.