The Nissan Titan and Titan XD haven't yet been rated by the EPA. In our real-world testing, we've found the diesel-powered XD to be particularly thirsty—but every truck in its class is the same way.
In combined city and highway driving, we managed roughly 18 mpg. That's not as frugal as a Ram 1500 turbodiesel, which can top 20 mpg, but it's marginally better than many diesel-powered heavy duty competitors.
We haven't had a long enough sample in the 5.6-liter V-8 to draw a meaningful conclusion. We'll update this space when we do.