It depends (TM)...
If the rear wheels are right at the back corner (less bodywork aft for detached flow to reattach on), I think skirts will have a bigger impact.
Also depends on the size of the vehicle's tire/wheel arch gap, the rim design (unless you've got smoothies on), wheel offset...
I agree with Frank: for a normal car, you likely won't be able to see the results over the usual tank-to-tank noise.
Also see:
http://ecomodder.com/forum/showthrea...-camry-84.html