I'm trying to find MPG fuel economy estimates for a 2005 GMC W4500
Diesel Cabover with a 14' box.  I can't seem to find published
estimates anywhere, and would like to have this information before I
make the purchsae.  Even if you are not able to find it for this
specific configuration (14' box), figures for the 2005 GMC W4500
Diesel would be acceptable, and data relative to other similar
models/years would also be acceptable (Chevy, Isuzu, etc).

The only place I could find anything on the diesel was here.  It is
for an older article (2001).

http://www.autoserviceworld.com/issues_SGM/PrinterFriendly.asp?story_id=SGM78519&id=67639&RType=&PC=SGM&issue=02012001
"average fuel consumption figure of 14 MPG in real-world mixed city
and highway driving"

I did find a few things mentioned concerning the gas versions.  They
seemed to run in the 10-12 mpg range.

I hope this helps you out some.

Thanks!