I looked into my
little book and found a short paragraph on deflectors. It states that these deflectors have the same effect as increasing the radius of the leading edges. In [2], Hoerner gives a reduction of Cx from 0.71 to 0.26 for deflectors on the leading edges of a bluntnose body, and a reduction from 0.58 to 0.27 when used at the rear. Unfortunately, these are only for lab models, and the rear mounted deflectors showed little or no improvement when used on road vehicles.
The effect on drag for front mounted deflectors depends on the aerodynamic situation between the tractor and the trailor. Buckley et al. ([1]) give a 0.05 reduction of drag for a cab with sharp edges and deflectors with 0 degrees of deflection (i.e. parallel to the direction of movement). This reduction increases to 0.25 when the angle is increased to 15 degrees.
In [3] Wysocki researched the aerodynamics of a long-distance bus. Other than changing the front angle and finding the penalty of using side mirrors (8%), he also tested a deflector on the rear, used to scoop fresh air down onto the rear window to keep it clean, finding it increased drag by about 3%.
[1] Buckley F.T., Marks C.H., Walston W.N.,
A study of aerodynamic methods for improving feul economy, US National Science Foundation, final report SIA 74 14843, University of Maryland, Dept. of Mech. Engineering, 1978.
[2] Hoerner S.F.,
Fluid Dynamic Drag, Hoerner Fluid Dynamics, PO Box342, Brick Town, N.J. 08723, USA, 1965. (I believe there is a new edition of this book.)
[3] Wysocki Z.,
Badania aerodynamiczne autobusu PR-110U Jelcz-Berliet - rozkłady ciśnień, Spr. Inst. Lot. 9/BA/78, Warszawa 1978.