My data with bike and rack

We used our old car for travel, never paid much attention to aerodynamics or consumption with our bikes on. Had purchased an aftermarket hitch for the 3 when we bought it. Today I did some testing to evaluate things for future road trips. Total trips were 17.5 miles, within an hour of each other, same route, same AP controlled speed. WH/mi data recorded along the way and upon completion:

Bike/Rack on versus off along the data points
465/368
426/320
361/261
364/264
322/234 end of trip.

Seems to me that this would cut our range by about 1/3rd, especially if I put the 2nd bike on. We could take off the tires and seats to improve a bit but what a major difference. I've read that the aerodynamic differences are worst at faster speeds so this would really limit travel with our bikes. Might have to get the CT and keep the bikes in the back...