GreenLight is an API that monitors and optimizes the fuel efficiency of truck fleets. It is a physics-based fuel and emissions calculator that uses state-of-the-art vehicle simulators to provide accurate calculations of fuel consumption and CO2 emissions. GreenLight analyzes historical fleet trip data and creates digital twins for truck analysis for accurate fuel and emissions analysis. Fuel efficiency can be achieved by selecting the most fuel-efficient alternative route for a given origin-destination and truck. Optimal combinations of trucks and shipments can reduce fuel consumption and carbon footprint. GreenLight enables accurate tracking of a truck fleet's performance by providing granular information on fuel consumption, CO2 emissions, and energy efficiency, and suggests optimal routing strategies to minimize fuel consumption and carbon footprint.