James, MPG's definitely a valid concern, especially since the reason I drive a small car is precisely for the MPG. I calculated the extra fuel cost per 400 miles with the guards on to be $8, or 2 cents extra per mile with gas at $2.50/gallon. I built the rig to be removable and stored inside the car. All of the guard panels will fit in the back seat area, and the long box beams will fit with one rear seat folded flat and having them pass through into the trunk. My plan is to transport the rig inside the car for the long trips to/from the Plains, as well as any long repositioning days while on a chase trip. I'll mount them in the morning of the first in a series of chase days, and remove them again at the end. Mounting/removal takes about 15-20 minutes by myself. At home, the guards can be removed and carried inside my apartment in two pieces.
Even if I chose to leave them on full-time on a trip, I figured the extra fuel cost for a round-trip to Elkhart, Kansas from here (1,400 miles total) would only run an additional $29. If I only mount them on tornado chase days, I'm looking at most an extra $8 per day on average. I expect to have them mounted maybe 10-15 days a year, for a total additional fuel cost of $150 or so annually. I look at it as a self-insurance premium against a catastrophic window loss, all while opening up more positioning and escape route possibilities around storms.