I take it your V6 is an auto from those rpm numbers?
The point about gearing is this... changing gear can waste fuel, and trying to accelerate at low RPM in a high gear is very inefficient (big throttle application, lots of fuel, little action). An example of that in a manual would be at 30mph in 4th... speed drops to 25mph, you're falling off the bottom of the torque curve and give it a big prod of the right foot to get back up to 30. In third, it doesn't require this, and doesn't sit at uncomfortably high revs either, and runs as efficiently overall. The advantage of this is that it's easier to pull back up to 30 when the speed drops and also you're more aware of the speed of the car and it doesn't run away up to 35-36mph and FLASH! Speeding ticket.
In the auto you'll find it doesn't shift into 4th until 35mph anyway.