It's often been asked "How does the BMW i3 REx perform when its battery is depleted?"
Well, if you're interesting in knowing its top speed when the battery is empty, then this video provides an answer - beginning at the :53 mark.
"German Autobahn - full throttle shortly before and while the battery is empty."
i3 REx owners largely confirm that 70 MPH is the max speed the car can maintain when the battery is depleted. If pushed beyond 70 MPH or so, the battery depletes itself even more, which leads to a reduced-speed condition where the car will begin to slow well below 70 MPH until it can replenish the battery enough to pick up the pace.