Sounds a bit like it could be a number of problems compounding together. If you can check and set the timing that would be a big help and remove a variable. When I got my truck the timing was retarded big time and it cost me and average of 4miles to a Imperial gallon as well as significant power. The smoke could also be an indicator of a timing issue or a worn fuel system. Stanadyne additive has also made a big improvement for me, I figure its getting me at least a 2mpg improvement.

In regards to calculating mileage. In order to get accurate numbers you need to start with a completely full tank / tanks. Keep track of mileage and all fuel added and make sure the tanks are dead full when you go to take your numbers for calculating. All fuel pumps are different and will shut off at different points. Some pumps will click off after adding twenty litres and require a tedious filling process, while some will fill the tank almost all the way. Regardless, I can always get in a minimum of 5 litres or more after the pump has shut off. These differences in fuel volume will cause huge discrepancies in mileage numbers.

If you have a smart phone check out the Fuel Log app, I use it with my truck and it saves a lot of time and effort hand calculating everything. It also keeps track of all kinds of stats and will calculated the cost to drive a given distance based on you averages and fuel costs. If your into that sort of stuff.

Sorry for the long winding story, I take my mileage calculations quite seriously.

 

Update:
Replaced the air filter, cleaned the cdr, replaced the fuel filter, and gave it an oil change. Sucessfully cleared up almost all my black smoke. now i only get a little bit at hard throttle/higher rpms.
BUT now i notice alot of grey smoke at idle. one whiff and you can tell its unburnt fuel. I think my injectors and or pump are done. over 200k and im betting original parts

 

Injectors for sure......

 

I'd try the old ATF soak treatment. Empty the fuel filter, fill with ATF, install, run for 30 seconds or so, shut off and let it sit overnight.

 

X2 on atf treatment!!!!