In recent decades, great improvements in vehicle efficiency and emissions reductions have been made. Changes that energy companies made to gasoline and diesel fuels enabled and enhanced some of these advances. Despite a much greater number of cars and trucks on American roads, the combination of technologies has resulted in improved air quality.

For example, fuel cell vehicles that operate on hydrogen are envisioned as one long-term alternative to conventional autos—if the remaining technical, safety and economic hurdles are overcome.

While time is needed to pursue such breakthroughs, the auto and energy industries are making advancements in the internal combustion engine, cleaner-burning and alternative fuels, improved emissions control devices, and new powertrain technologies such as hybrids.

Also, U.S. refiners have made many major changes in gasoline and diesel over the past three decades, which have contributed to improvements to air quality. As fuels have become more complex, refiners must provide nearly 20 different types of gasoline formulations to meet federal clean-air standards in different areas of the country.

One key area of change has been in the reduction of fuel sulfur. And as crude sulfur levels continue to increase, refiners are investing approximately $8 billion in additional sulfur removing hydroprocessing technology to meet this decade’s new gasoline and diesel fuel (Tier 2) specifications.

For example, a new car using Tier 2 gasoline and equipped with the most advanced emissions reduction technology will:

• Produce 99 percent fewer emissions than a new 1970 vehicle

• Reduce nitrogen oxide emissions, a key precursor to smog, by more than three million tons in 2030

• Be equivalent to taking 164 million cars off the road

Numerous other enhancements to the quality of gasoline were introduced. Lead was phased out, vapor pressure reduced, oxygenates added, Phase I and II federal reformulated gasoline (RFG) and California Cleaner Burning Gasoline (CBG) were all successfully introduced and sulfur was removed from both gasoline and diesel fuel.

Read more about fuel choices and transportation technologies.