Truck Slipstream Effects?

[MSI]

There have been some experimental tests run on the effects of vehicles in proximity to trucks.
A quick look brings the following reference: Aerodynamics of Passenger Vehicles in Close Proximity to Trucks and Buses
Robert Heffley, Systems Technology (STI), SAE paper 730235
The full report was performed under contract FHWA-RD-71-3, October 1971, STI Report No. TR-1016-1
I believe you can get the full report from STI http://www.systemstech.com at nominal cost