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I've heard some SUV and PT cruiser owners report that when using a luggage box or platform with a large tote on on their hitch they have seen improved MPG...
This is due to a couple of effects I think, first the surface about halfway up the back of the vehicle allows for a more sedan like clean separation of air flowing over the top. This improves form drag or shape drag, due to allowing the air to depart the vehicle at a more horizontal angle, thus effectively "tilting the airfoil" that the vehicle body forms forward and causing the shape "drag" or aerodynamic lift to act in a forward direction.
Secondly, the sides of the box being narrower than the vehicle, at the minimum so that the lights are visible, creates a step that will fill with a fairly stable vortex, which has the effect of rounding the back of the vehicle and allowing more air from the sides to tuck around behind it. This reduces the base drag. Some of this type of effect may also be seen on the top as well, dependant on how the tailgate/roofline curves down.
As for just a flat plate... it's usually more beneficial to encourage air from underneath to flow upwards behind the vehicle. This does make for more spray on the back window if not balanced by flow from the top but it also helps the bulk airflow to be generally more horizontal than downward. Therefore a lower down plate or hitch load platform with nothing on it, may be more likely to have a detrimental effect. Raising it to around half the height of the vehicle would allow it to act like a splitter and separate air off the back cleanly.
It's possible that rear tire carriers can have some of these effects, but they may be a little lopsided if carried off center or absent if the tire is on the small side. For instance on your Liberty, you might be getting some drag reduction on the passenger side, but I think the amount it extends from the back is too short to have much effect on air coming over the top, and it's too far from the drivers side. I wouldn't be surprised if you told me it tends to pull right slightly at speed.
On that vehicle, I might be inclined to investigate making a lightweight box affixed to the tire carrier, that is about the size of the tire top to bottom, about twice as wide front to back and fits between the lights/bumper ends.... though maybe a little shorter than the tire, so it's poking out the bottom and top... guess it could be mounted on the hitch and be a kind of shell that slips over.
Hmmm thinking on some rule of thumb stuff I hashed out a while ago, if you can get about an 8"-10" step on the sides, and then have a second box inset another 8"-10" inches, with the top and bottom left level, you'd have a sort of boatail round the sides where vortices would round out the effective surface.
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I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
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