Help me understand how these work (or don't)

I've noticed that a lot of newer cars include small plastic tabs in front of the wheels, almost mini-airdams. These are on my BMW 128i:

I'm assuming these are designed to improve aero efficiency by steering air away from the spinning tires, but exactly what dimensions they should have, or any other effects they might impart, are a mystery to me. So, lacking any real knowledge, I decided to fab up some for my Volvo V50, which is not exactly the most efficient highway car and could use some help. Whether I've actually helped is why I'm here.

So this is what I've done. Small tabs in front of all four wheels, plus a small airdam at approximately the same height (the gaps between the two are to allow air to flow to the factory brake cooling channels; I'm not wedded to keeping them open if it makes a huge aero difference, as this is not going to see lots of heavy braking). They are made of ABS plastic and are quite rigid.

Front:

Rear:

The tabs extend up the wheel arches due to 1) the curvature of the body, and 2) doing so allowed me to use factory screws to attach them. I can, of course, trim them to conform to the curvature, but I figured let me ask before I do something irreversible.

So what have I done? Should I expect this to make any difference in aerodynamic efficiency at all, good or bad? Anything I should change in the design? Or just rip them off?

Doing a little digging and I found this thesis that includes some good information. The math is way beyond me, but there are clear descriptions and diagrams that make sense of at least the basics.

Edit: I'm going to link to any other papers and articles I find in this post.

Assessment of conventional and air-jet wheel deflectors for drag reduction of the DrivAer model: suggests that conventional wheel deflectors may be of limited utility, especially on aerodynamically efficient vehicles

Drag reduction by application of aerodynamic devices in a race car: not necessarily applicable here, but I figured it might be of interest to some

I'm interested in your results. 

Reading the linked paper more closely, it seems that on the model in question, after testing five heights for front deflectors (rears were not tested in this paper) between 20 and 60mm, while larger deflectors had lower Cd locally, the overall Cd for the vehicle was lowest with the 40mm deflectors due to changes in pressure at the rear of the car. The deflectors I installed measure 35mm front and 45mm rear at the shortest point (fully inboard), with both 150mm wide. For reference, my 128i (lower and smaller than the V50) measures 35mm front and 25mm rear, while my parents' Audi Q3 (taller, but also more similar in shape to the V50) measures 25mm front and 45mm rear. This suggests that I'm at least in the ballpark for height.

What I can't find any information on is how far toward the outer edge of the wheel the deflectors should extend. The paper linked shows them starting at the inboard edge and extending roughly 2/3rds of the distance across the wheel, ending before the bodywork turns upward. This is also the case on the other vehicles I looked at, and from other images found online. What I don't know is if further extension toward the outer edge is aerodynamically detrimental, or if this is an aesthetic choice, or some combination of the two. Trimming mine back to achieve something more consistent with what I'm seeing on production vehicles is easy enough, but I'd rather not do it until I have a better sense of the aerodynamic impact.

Edit: I found one discussion (not peer-reviewed) suggesting ~100mm inboard and ~100mm above the ground as fairly standard position for fixed straight deflectors. Mine are certainly further out than this; I would have to take about 40mm off each to get them 100mm inboard. The fronts are 125mm from the ground, and the rears 115mm, so a bit more than is apparently typical.