A report, prepared by Tiax LLC, a leading product and technology development firm, estimates the life-cycle costs for heavy-duty diesel and natural-gas vehicles that meet 2010 EPA and California Air Resources Board emission requirements. The report, "Comparative Costs of 2010 Heavy-Duty Diesel and Natural Gas Technologies," looks at refuse haulers, transit buses, and short-haul trucks.

Heavy-duty natural-gas-powered vehicles that meet those standards are more economical to own, operate, and maintain than comparable diesel-powered vehicles when the price of crude oil exceeds $31/barrel, the study found. And oil prices have been well above $60/barrel for some time.

The study is based on a life-cycle model that incorporates fuel, operation, and maintenance costs. Variables include the cost of crude oil, type of exhaust aftertreatment system used, price of natural gas versus diesel, price of liquefied natural gas versus compressed natural gas, engine costs, and fuel economy.