I'm not an expert in California standards, but generally I can say that the California standards regulate GHG emissions, while the U.S. standards relating to the fuel efficiency of cars or light-duty trucks are based on fuel consumption, so there are two different ways.
If you deal with fuel consumption, you deal mainly with carbon dioxide, the greenhouse gas emissions that are carbon dioxide. That gas forms 95% of the gas emitted with fuel consumption, so it's almost a 1:1 relationship.
The California standards regulate all the GHG gases—carbon dioxide, plus two or three other gases—so that's the distinction between the two. The U.S. national administration regulates what they call “fuel efficiency”—it's miles per gallon—while in Canada the national legislation we have that hasn't been proclaimed yet would also regulate fuel consumption.
I hope that clarifies things a bit.