No mythbusters doesn't break the law when doing certain tests which would be the speed limit, not breaking the speed limit where aero has no major drag on a vehicle and you don't get any amazing results because the air drag isn't all that important to hurting the vehicle at that speed.
I know that there test with the p/u bed is not correct and is simply because of the lack of aero drag, here's why.
I usually drive everywhere at 75 mph, i have a slightly modified dakota, very similar to the one they used except i have a few other items added to and blocked areas to increase my mpg.
At 55 and all my aero tricks i can and do 29 mpg, at 55 and no bed cover it loses maybe a .1 gallon..
Now at 75 mph and all the aero tricks i can do 24, remove the bed cover and it's all over down to 17, that is a HUGE difference, remove the other aero diversions and at 75 it'll get to 14.
Now here's the other 3 mpg loss, another member brought up playing with a sheet of plywood, and for a sheet of plywood it would be a large frontal area of drag, but aero doesn't work like that, the bumper protrudes and it 1st deflects and directs air, the hood then deflects and directs and then the windshield area.
The other 3mpg is from blocking the openings in the front of the dakota and air dam, i gave the air no pockets or area for it to become blocked and added a better diversion.
So even though if you stand back and look at the front of the jeep and stand a piece of a plywood and cut it at the 5' mark and THINK it's that aero dirty, it is not because the jeep doesn't meet that air like that as it goes thru it.
My dakota, with it's blocked grills and air dam would be by according to that theory aero dirty, but aero doesn't work like that and instead it is better directed and flows now.
Same with my GC and it's air dam, the air dam doesn't make it worse it makes it better by directing it away from areas of further drag.
At the end of the day the only simple help is over inflation, better oils thruout. Every other piece to increase mpg is going to cost money and then it will slowly have a return, lighter wheels, lighter driveline pieces, tires, blocked areas, exhaust, many other things.
1995 Jeep gc ltd, 2005 Jeep gc ltd , 71 Dart, 73 RR 440-6pk, 78 Mag GT, 85 Power Ram W-350, 92 Daytona, 99 Dakota, 08 Ram 2500
Less than 8 is like playing with half a deck
Long Island Motorsports Association