A 1-2mpg loss with a bike on back isn't much. Go ahead and remove the front wheel, turn handlebars flat and recenter as suggested it can't hurt. But don't expect to get back to baseline. The bubble behind your tailgate isn't static. A pair of eddy currents rotate in 3D with mirror symmetry down the vertical center plane of the vehicle. Balance dictates forward flow up the center then outwards towards each side. A smooth tailgate facilitates that flow bubble. A bike impedes that flow. Satisfaction follows expectations.
