GM's streamlined trailer
 

I GOOGLED IMAGES : GM MESA Proving Ground testing.A few pages down,there was an image of a 2005 Corvette undergoing cooling tests,pulling a 'towing dynamometer'.
The link is to :
HowStuffWorks "Corvette Specifications"
and maybe:
auto.howstuffworks.com>...>SportsCars>Corvette>200 0s-
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There is the gap in between,but you can see the difficulty they went to in its form.Also,if you look at the Proving Ground site,you'll see additional dynamometer trailers which are not as aerodynamically clean as the one behind the Corvette.

...here's an aerial view of the GM / YPG test track areas:

http://wot.motortrend.com/files/2009...53-623x389.jpg
GM and Army open up new test facility on Yuma Proving Ground — Autoblog

...this picture (2005) was taken at the "old" GM Mesa, AZ, track:

http://t1.gstatic.com/images?q=tbn:A...D7XOKATrUByn3w