Sorry for this but I need help with my 1992 K3500

did you change out the tbi gasket , unless you have the metal coated one this could be the problem ,it might be sucked in . also try using the middle grade fuel to see if the knock sensor is retarding timing . I have been fighting fuel millage problem with my 4.3 and just fix it by accident, it went from 20mpg to 16 for the last 8 years ,after replacing the intake gasket and major tune up with a fuel pump, the problem started with the 10% alcohol fuel ,so last week our gas station was out of reg so I put in the middle grade fuel and first trip out I noticed more throttle response and in the end went up to 24mpg first trip out. It has stayed between 23/25mpg on hwy and 19 in town I'm finally happy because this is what the blazer should be getting also my 92 suburban with a 305 pulls down 22/24 on the highway. ALSO THE STOCK GEARING FOR YOUR SET UP IS A 4.10 REAR 5.7 TBI WITH AUTO./THE STICKS HAD 3.73 GEARS.i THINK THE BEST YOU SHOULD SEE IS 14/16 MPG WITH THE O/D TRANS ON HIGHWAY