If the car is out of gas, but the computer in control hasn't figured it out yet, you can use the whole 0-40 miles per hour EV range.
Once it gets registered in the computer that you are indeed out, then the car enters "limp home mode" and you get to use EV only in the 0-22 miles per hour range. Maybe 25 MPH if you are going downhill!
I agree it allows you to drain the battery lower than normal limits.
It is most likely you will get 0-22 miles per hour when you run out of gas.
Gary's case was very lucky, and not typical, but I've heard it happen a couple of times.

-John