The regulations call for nitrogen oxide (NOx) emissions to be reduced by 90 percent, and to achieve that, GM had to make significant changes to the Duramax. The company was able to use some carryover components of the 6.6-liter, but about 60 percent of the parts are new. GM's approach is similar to the one Ford took with the 2011 Super Duty's diesel--to comply with the new regulations, the Duramax now uses an updated exhaust gas recirculation system (which reduces the amount of NOx produced during combustion) and a selective catalytic reduction aftertreatment system. With SCR, a small amount of urea-based diesel exhaust fluid is sprayed into the engine's exhaust gases. When the fluid mixes with the hot gases, the urea is converted into ammonia. The ammonia and a catalyst convert the NOx into emissions-friendly nitrogen gas and water vapor. The more demand put on the engine, the more DEF is used. The system also uses a diesel particulate filter to trap soot, and the filter regenerates about every 700 miles.

Expected range between DEF refills is about 5000 miles. When the DEF tank starts running low (there will be warnings), speed is limited to 55 mph, and when the tank is empty, you won't be stranded--but the truck won't go any faster than five mph.

Also included as standard equipment with all Duramax diesels is an exhaust brake. While pricing hasn't yet been announced, we anticipate the new emission-control systems will add a slight premium to the cost of the diesel engine. Fuel efficiency is also said to be higher than with the previous generation's engines; that, plus the larger fuel tank (now 36 gallons as opposed to 34), means the truck's range is as high as 680 miles on a single tank of fuel. The EPA doesn't rate trucks this heavy, but that range and tank size suggest this Chevy should achieve about 19 mpg on the highway.