Since they are easy to make and design, there are several 3d printable models. They are also very cheap so you can make dozens for a few dollars.
Samm Shepard 3d printed vortex generators and made road tests with good results. He also explains a science and aerodynamic principles behind it.
He used the F86 vortex generator:
Kyle also does a great job in simplifying the science behind it and also has real-life tests with positive results but claims they are only useful on high performance cars.
Autospeed magazine did some testing and achieved positive results in 2006:
Vortex generators on cars can achieve measurable, scientifically proven improvements in car aerodynamics – reducing both lift and drag. The Lancer Evo is probably the shape of things to come – expect vortex generators (of whatever design) to be used on new cars, especially to improve the flow around corners towards the rear of the body.
Positive gains can be achieved only by practical experimentation. That’s the downside – the upside is that the vortex generators are easy to temporarily stick into place with masking tape, and just as easily removed if they are not achieving the desired results. They’re also cheap enough that buying ten or so for experimentation is a good investment.But they did have some negative results if they were placed on wrong places.
GFY racing uses 3d printed vortex generators on race cars:
One of the commercial vortex generator producers, AirTab, did some testing and they achieved significant improvements on trucks. Due to their box shape, their aerodynamics is very bad.
Several research papers, like this one by MIRA show that the fuel savings can reach up to 3% with installed vortex generators.
There are simillar 3d printable models like this one: http://www.thingiverse.com/thing:1251022
Here are vortex generators mounted on a Sprinter van:
And user experience of RV owner:
Since vortex generators are easy to 3d print and easy to install and uninstall they could be great gadget to experiment with and see if you can save some fuel.
If you want to learn more on how to increase fuel efficiency of your car and get more MPGs here is a modder community with tons of useful posts and articles:
They use 3d printers also to make custom car parts:
... like this custom shift knob with engine cut off switch: