A failed modification (partial rear undertray)
 

My car has a rear bumper which has a lot of empty space inside and is not closed off at the bottom, so I thought I'd try covering some of the gap at the bottom, a kind of partial rear undertray, with some plastic sheet which I screwed to the lip of the bumper and fixed in place.
I didn't cover all the gap, but just what I could do in one go.

My mpg went down from 40 to 38 or thereabouts, recorded over several months.

I recently removed the piece of plastic to see if anything happened. My mpg went back up to 40.

I'm guessing that the partial covering of the gap created more drag instead of reducing it. (there may have been other factors, like the weather, but I think everything else was consistent)

I suppose the moral of this tale is, don't do anything by halves! And this A-B-A testing that I've read about is a good way to prove a point one way or the other.
:)

Temps track it too?

I don't know how secure the "plastic sheet" was supported or if there was a gap at the leading edge that wasn't sealed to the body? Is it possible you created a parachute that only bulged out at highway speeds? At any rate let's not make the blanket assertion that partial belly pans are bad, only that in this case it deserves more study.

My first belly pan prototype wasn't secured at the leading edge quite securely enough and was literally blown off by a gust of wind. (I was testing it on a deserted road at the time) Version 2.1 is very solid and has withstood being highsided on packed snow with no damage.

On wagons and fastbacks a diffuser at the wrong angle can increase drag. Assuming your car is about 0.3 Cd it is possible that a badly engineered diffuser could take away those 2mpg.

http://i43.tinypic.com/9u427p.jpg

It was secured at all edges and quite neat (I can't upload any pictures at the moment, sorry). I don't know if the weather was a coincidental factor, it may well have been.
This wasn't a very scientific test!

A few years ago I did a mod to keep dirt and salt from getting kicked up on top of my gas tank from the rear wheels. I wondered why they left this open at the factory. I found out, I lost 5 MPG closing off the rear fender wheels at the back. The pumping action of the tires makes the fender wells a high pressure area.

When I put in my rear bumper mod (mine has to cover the gas tank) I left a tunnel on both sides to let the wheel wells vent. I saw a slight increase, and dirt doesn't pile up on the gas tank.

This summer I will try to dial in the rear angle by raising or lowering the rear springs.

Good point. The angle may have been part of the problem.

Thx from this info, very useful!

This brings me a question though, if I put mini kamm, isn't that making it less like a wagon and more like a hatch back?

Diffuser would then work better at angle like what they found out with A2 modification that 2 degrees was optimal when adding those pressure things to rear of car?

I believe that making diffuser at 0 degrees would at least not harm compared to there being parachute like form of rear bumper without anything there, I guess that is common to all wagons with spare wheel under the trunk floor.

Exactly! Treat your car as the wind sees it. If you have a Kammback right now, make a diffuser as if it's a wagon.

The problem with just blocking from the rear axle to the bumper is that the angle of those parts may be 10* or so, playing havoc with any body form.