Until I read jaypeabey's comment pointing to a series of articles on the AutoSpeed blog, I had no idea that a bunch of commercial products similar to GasPods already exist, and that they're well-known in aeronautical applications, too.
VG Fuelsavers appeared on The New Inventors in 2006. AutoSpeed contacted the Fuelsavers people shortly after that, and offered to actually, you know, test them, which The New Inventors doesn't do.
This offer was, silently and mysteriously, rebuffed.
The Fuelsavers site claims a "6% to 9%" reduction in fuel consumption", which is plausible, if you do almost nothing but highway driving.
Even a 6% gain for a car mainly driven in city traffic, though, does not seem likely to me. Even at highway speeds it's difficult for a drag-reducing aerodynamic modification to give a fuel-economy gain of more than about 60% of the drag reduction. Since drag increases with the square of speed, aerodynamics are very important to racing cars, and moderately important for highway driving, but almost irrelevant at low speeds. (This explains why you don't see a lot of aerodynamically-designed bulldozers.)
These ones are called "Airtabs". They may be the first gizmo I've ever seen that claims some connection with NASA, and is actually telling the truth. (See this, for instance, for the usual situation. Or this, for the similarly-common military version. Some people, though, will believe anything.)
AutoSpeed tested the Airtabs, but not very well. The test wasn't blinded or well-controlled, and the only test vehicle they actually measured fuel consumption on was a Honda Insight. If it was the first-generation model, then it started out with a coefficient of drag of only 0.25. That's about as good as production cars have ever managed, so it's arguable that it can't be improved very much more, and certainly not by just sticking on some little fins.
To be fair, a facility that can do proper drive-cycle tests probably can't do them on aerodynamic devices, because drive-cycle tests are usually done on a stationary dynamometer. You need a wind-tunnel to test aerodynamics thoroughly, and they're a lot rarer than dynos.
But, as AutoSpeed points out in their first article about vortex gizmoes, you can get a good idea of the structure of the airflow over a car by sticking bits of yarn all over it. And it's also possible to get decent numbers, by doing the rolling-down-a-hill-in-neutral test I mention in the Gaspods post. You can even do it with only one test car. And if I were doing it, I'd start with a "normal" car with a CD of 0.3 to 0.35. It would also be instructive to test a vehicle with quite lousy aerodynamics, like a van or pickup truck.
(You can actually even estimate your car's coefficient of drag by rolling in neutral.)
These are "Aerotech" vortex generators, sold in sets of 50 for truckers. The rectangular-prism end of a truck trailer is an aerodynamic disaster area, and fuel economy is something of an obsession for many truckers. Anything that reduces drag even a little bit for a long-haul trucker is likely to be worth quite a lot of money; the Aerotech page claims an improvement of "as much as 1%".
For a car, that's not worth paying for, which presumably is why sellers of vortex gadgets for cars tend to be more... optimistic... about their products. One per cent is worth paying for for a trucker, though.
Note that there are also "vortex generators", also known as "turbulence generators", that claim to create a vortex in the air going into the engine, rather than the air flowing over the car. Turbulence generators have been sold in umpteen forms over the years, and have never done a damn thing, except they often do restrict airflow into the engine and thus reduce its maximum power.
This actually often will save some fuel, because now pushing the accelerator all the way to the floor will only give you, say, 80% of what full throttle used to be. Just not pushing the pedal to the floor will do the same thing, though, and still let you have all the horsepower you paid for when you want it.
In light of the panoply of aerodynamic, possibly-actually-effective vortex gadgets on sale, I clearly should have done more research before writing that blog post. As, of course, should the journalists who wrote those happy-clappy articles about the GasPods, never mentioning that they're not actually a new idea.
As that article jaypeabey linked to says (quoting the Bosch Automotive Handbook), you can reasonably expect a given reduction in drag to give you a bit more than half as large a reduction in fuel consumption, at highway cruise speeds. Quite a bit more than half as large a reduction if you're driving really fast, legally or otherwise; no gain worth paying for if you're driving much slower, in traffic.
This is what AutoSpeed found in their dodgy test with the Insight, and, as I said in the GasPods post, there's no strong reason to presume that any of these devices even can somehow give you a larger fuel-economy gain than the drag reduction they deliver.
They're not snake oil, but they do seem to me to be rather oversold.