Welcome to the forums dfunk.
While I am not going to say it is impossible the fuel economy goal for the truck is not realistic. Nothing is impossible if you have enough time, money and patience.
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First up some links from the EPA fuel economy guide:
This most likely is similar to your vehicle.
http://www.fueleconomy.gov/feg/findacar.htmHere is link of a 2010 GMC it is not an apple to apple comparison. However, it gives an idea of the amount of the increase in fuel economy in trucks over the years.
http://www.fueleconomy.gov/feg/findacar.htmI am no engineer by any means so that is my disclaimer
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. There are several concerns to contend with.
First what is the primary purpose of the vehicle? Is it a truck used for hauling? If so that my be very problematic to increasing fuel economy.
Rolling resistance is something that will need to be optimized. If the truck is a traditional 4X4 it take substantial amount of power to turn the gear train. Almost always a 2wd will have better MPG than a 4wd.
Weight is another concern. Trucks are built sturdy (most of em) and there fore weigh considerably more than traditional cares.
Aerodynamics are another concern. If I recall correctly the 87 GMC is very boxy. The harder it is to push it through the air the more fuel is consumed.
Fuel distribution and control could be a good place to start. Many of the improvements in modern vehicles are derived from this. If your truck is a TBI upgrading to a multiple port injection would be a good start. Edelbrock use to have a TBI ot MPI conversion. I do not know if they still do. Peruse through this link for what is current.
http://www.edelbrock.com/automotive_new/mc/efi/pf_intro.shtmlThere is also a huge offering of stuff directly from GM:
http://www.gmperformanceparts.com/Parts/showcase.jspThis would just be a start. I would ask you the same question I often ask customers wanting to fix noise and other unusual things.
What is the dollar to aggravation ratio? In you case what is the MPG improvement to dollar ratio?
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Good luck with it what ever course you decide to pursue we might be able to help out some.
Sam