Abstract This paper investigates methodologies to quantify CO2 emissions from cars and light trucks in Florida. The most widely used methodology to calculate greenhouse gas emissions in the transportation sector at the local level uses a harmonic average (HA) methodology based on nationally averaged fuel economies that assume 55% city and 45% highway VMTs. This paper presents a local condition (LC) methodology that accounts for county-level variations in city and highway VMTs, as opposed to assumed uniform driving conditions. Both HA and LC methodologies were used to estimate and compare absolute and per capita CO2 emissions both statewide and counties for 2000 and 2008. From 2000 to 2008, statewide absolute and per capita CO2 emissions increased similarly using HA and LC methodologies; however, the percent change varied considerably among counties. Statewide CO2 emissions calculated from HA and LC methodologies differed by only −0.2% (2000) and 1.7% (2008); however, the differences in the county-level emissions ranged from −8.0% to 14.9% (2000) and from −5.6% to 17.0% (2008). While either the HA or the LC methodology yields a similar result statewide, significant variation exists at the county level, warranting the need to consider local driving conditions when estimating county-level emissions.