127 Miles on nearly 20 gallons?

[Buck Rogers]

i recently started noticing the gas mileage going down
on my car, first i was getting about 400 miles out of
a tank which is about right, but then i noticed i was
only getting about 230 the next month and then of
course right now i just got 127 miles on a full tank
on my 84 4k. it also blows black smoke and bogs down
really bad going into 2,3,4,5 gears, the bigest drop
of course is from 2nd to 3rd! if anyone knows what it
might be, thank you!

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