One of the major sources of wheel drag appears to be caused by jetting vortices exiting out and up from the tire/road intersection. This was first mentionioned by Morelli (he of the Aptera bodyshape) citing Cogotti, whowas able to achieve a power reduction of 5kW for and expenditure of 240W by adding in-wheel turbines to the rear wheels, and directing the flow to counter the vortices (SAE 2001-01-0491 it'll cost ya)
More recently Tamas has produced a flurry of papers, all of which are freely available as PDFs online. Here's a couple.
http://www.cfd.hu/pdf/10%20Dr.Regert...2010.11.11.pdf
https://www.vki.ac.be/download/defen...sum_regert.pdf
The first one seems to show some promising techniques for simple add-ons, in other words, Ecomods.