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.

There are 4 variations of the design, each pair includes a plain version and a valve-hole version. Print four of the valve version and 16 of the plain petals. My new favourite is the Mirrored Vent version. It looks pretty good and has two benefits (a) it lets a little air in to cool the brake pads, and (b) it may reduce drag even better than the plain inserts! See Alexey Vdovin thesis Investigation of Aerodynamic Resistance of Rotating Wheels on Passenger Cars.

My tests with the original plain inserts 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 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.

Version 16 is older and simpler.

Later versions have been improved by Erik Kamila (https://pinshape.com/users/143267-birdbone) who chamferred the edges for us. He also has designs for different Leaf wheels - well worth a look.

Version 23 "Kermit" features ventilation slots that look a bit like Kermit the frog eyes. 

Version 24 "Curved vent" and "Mirrored vent" feature ventilation holes placed at the axle end of the insert. This should provide ventialtion and optimal drag reduction - as described in a thesis by Alexey Vdovin Investigation of Aerodynamic Resistance of Rotating Wheels on Passenger Cars.

Limitations: 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 since then. 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 problem with PLA. 

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

Update 29 March 2020: Since I started using these I've lost a few and replaced them all with newer version. My car is currently running the Version 23 "Kernit" design which looks a bit more interesting than the vanilla Version 16 variant. I;ve just read this thesis https://core.ac.uk/download/pdf/70600281.pdf which prompted the new vented variants (Version 24). I'll try those next.

Update 24 April 2020: I've just printed 5 petals of the Mirrored Vent version and it is now my strong favourite - see new main photo. 

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 printing 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!