This is a simple aero mod for a Nissan Leaf alloy rim. It's designed to clip into the rim hole and thereby reduce aerodynamic drag past the wheel.
I've added a variation with a small hole where the valve protrudes. Print four of these and 16 of the plain petals. This will make it easier for you to know where the valve is for when you want to pump the tires up.
My tests indicate improved efficiency of 6% when travelling above 95 kph (59 mph), and 3% improvment at 80 kph. [See my coast down test details.]
The insert clips in place with a satisfying snap. You can remove them using a screwdriver or fingernail to lever the inside (or outside) edge. Once you have one rim hole cleared you can also poke your fingers behind to push the clip out of the next insert. [Don't do this if the brakes are hot!]
They weight 27 grams (0.95 Ounce) each or 540 grams (1.2 pounds) for a full set of 20.
Limitations: I don't know how long this print will last outside in the sun and rain. I printed mine in black PLA in October 2018. It would probably be better to print them in ABS to withstand summer heat. The black colouring has faded a little in the 6 months since I've been using them. Research seems to indicate PLA is more UV resistant than other plastics though. (Also: PLA is "biodegrable", but only when exposed to continous 60+ degree C temps for ar least 5 days straight. So "wilting" is likely to be the more likely issue.
Is it worth it? If you have a 3D printer, then yes.
Update 14 Oct 2018: I just went on a 757 km (470 m) road trip with the inserts and one of them dissapeared somewhere. I have tweaked the design a wee bit to improve the stiffness.
Update 8 Apr 2018: The current iteration (version 16) has extra strengthening and seems to be holding up well.
Update 6 June 2019: I conducted some coast down tests to compare performance with these rim inserts (and the front grill blocks) verse without. TLDR: these provide an improvement of 6% at 95 kph (60 mph) down to a 3% improvement at 80 kph (50 mph). At speeds higher than 95 kph the improvement will be higher still, and below 60 kph they won't have any meaningful effect. Full details of my tests here: http://www.myonlinediary.com/index.php/Energy/CoastDownTest
A note about fair use of this design: If you already have a 3D printer and an early LEAF then printing a set for your self is worth it. However these take forever to print so it's not easy to make money 3D pringting them for others. If you want to give it a go - then knock yourself out. You can copying and sell them for money - that's fine by me. Good luck!