So i completed a number of trips to confirm my results, the total range with my particular box. The result was that consumption went from 3.1 m/KWH to 2.6 m/KWH. The predicted range was 200 miles. Data thats important. I was using the official ID4 roofbars, with a Thule Pulse L box --- Thule Pulse
For 240 of the 300 or so miles I kept a steady 65-75 mph speed, with the AC on. There was approx 150lbs of gear in the box. One leg of the trips was just bloody awful traffic, with some higher speed moments. There was a good mix of North/south/east/west to create variables with wind. (I think new EV owners are going to be surprised how much of an impact wind has on range)
Yes I could use a smaller box, but thats the one we use with the Pacifica Minivan. I have done 1000's of miles with the roof box on that vehicle, and record a drop in efficiency of about 17% with that combo. That vehicle is noticeably less aerodynamic.
You can learn more here, and hear a rant or three about california drivers here