The MPG value is also adjusted slightly to account for differences in test procedures made since the base year. The MPG value is not adjusted for in-use short fall. Since this is an approximate calculation, the actual gas guzzler tax may be off by one tax bracket. Full fuel-cycle estimates include all steps in the use of a fuel, from production and refining to distribution and final use. Because of this they give a more complete picture of the impacts on global warming of use of a particular fuel.
Some gases have greater impacts on climate than others.
Scientists have developed relative global warming potential numbers for each gas that gives its impact on global climate realtive to that of carbon dioxide. We use the global warming potentials developed by the Intergovernmental Panel on Climate Change to combine the three gases into tons of carbon dioxide equivalent. We believe this gives the most compete and accurate reflection of the impact of each vehicle's energy use on the global climate. Numerous assumptions and calculations are necessary to estimate full fuel-cycle greenhouse gas emissions in carbon dioxide equivalents. Because of this, estimates from different sources will vary.