Here are a couple of things to consider about the improved gas mileage. These are my semi-educated assumptions...
1. The power to weight ratio has improved alot. So the VK engine is working much less than the VQ to move the Pathfinder around.
2. The VQ40 is designed to run at higher rpm's in order to get the most power from it. I noticed immediately that the VK runs at a lower rpm at highway speed than the VQ did.
Oh yeah, I've noticed my fuel mileage improvements despite
my heavy footed fun factor with the new power!!