Alrighty, I raided a folder at university. Shame I don't have any electronic copies of this stuff but you might want to try looking for work by:
Stapleford and Carr (1970) - multiple wheels, moving ground wind tunnel results vs. stationary ground
Fackrell et al. (1972, 1973 and 1975) - as has already been mentioned. His results are most often returned to by those doing simulations on wheels as a means of validation of the comutational results.
Imaizumi and Yohida (1991) - did experiments on different types of wheels, and specifically on open wheels and partially-enclosed ones (sedan style car).
Oswald and Brone, 1981 as well as Mercker et al. 1991 - studies on enclosed wheels.
Hinson (1999) - experiments on an open wheel conducted for BAR F1, for a range of speeds.
Whitebread (2000) - improved on Hinson's work and looked into the effect of wheel yaw.
Knowles et al. (2002) - made detailed laser doppler annemometry measurements of the wake of an isolated wheel, specifically with regard to the vortex structures.
McManus and Zhang (2006) - detailed computations of the unsteady flowfield around Fackrell's wheel.
Couldn't tell you how useful these papers are to any of you but if you're willing to do some sniffing to track them down they might make for some enlightening reading. The last two probably contain a good summary of what's gone before but a much enhanced understanding of the flow behind and around the wheel just by virtue of up-to-date techniques and standing on the shoulders etc.