This is a odd one. So I've owned a few 4.3 vortec trucks before. I have a 01 blazer 2door LS . I Just got from Florida , super clean runs seemingly well no check engine always a city driven supervisors truck to run out to job site for city maintenance company. I did a half tank fill 20$ reset the odo and got 90miles till empty 👀🤔 . Instantly said that doesn't quite right . And it runs like a top.

What would some of you suggest on a tuneup/change parts wise? Realy that would pertain to improving gas mileage. I know minor stuff like dirty maf can make all the difference in fuel and I was thinking fuel filter .
Any other idea would be helpful. Pretty much bought the blazer 2 door specifically cause I wanted one too save on gas over Driving a Daily 94 Suburban gas hog 5.7L
The 4.3 is technically a 350 with two cylinders chopped but I should be getting a hell of a lot better mileage then that even if I recall the last vortec I owned.

 

Around 20 mpg in a good mix with highway at moderate speeds.

I would suggest an extended service, maybe even changing the coolant and a flush.
Oil, oil filter, fuel filter, transmission pan removal and transmission oil filter - top off.
Check base ignition timing, check spark plugs (all, even if it is a PITA).
If you can, and have a proper spark plug adapter - compression test as all plugs are out. (I'd remove the fuel pump fuse for that so it doesn't flood the engine)

This will give some good overview and make sure you have all the OEM stuff in place. From there drive the Blazer and monitor fuel consumption vs. driving habit. (city, highway, off-road, etc.)

 

4WD is 14 MPG

 

In town my 4wd gets about 14 mpg and on the highway I might get 19 if I'm at 60. I also have 3.73:1 gears.

The higher geared 2wd Blazers got upward to maybe 24 mpg on the highway?

 

Having seen issues like this in both a 4.3 and 4.2 engine. Check the fuel pressure the ECM will compensate for a failing fuel pump and with small leaks the engine won't set a code.
Using a scanner check the fuel trim and timing and CMP retard. Also check the O2 responses as a lazy O2 sensor will cause issues.

Then the standard villains would be next plugs, wires cap and rotor but as a fleet/city vehicle I expect the maintenance would have been done. These trucks are picky about what they like.

Then start with the weird thing like have the changed the tire size? Would affect shift points and speedometer reading and mileage count.

Half a tank on a 2 dr is 9.5 gallons so you are getting less than 10 mpg.

 

Other thought does the fuel gauge work right? My ZR2 it reads lower than the actual level.

 

Try doing your mpg calculation from a full tank, rather than guessing from a half tank. Every GM vehicle I've ever had, and I have had a few, seems to get great mileage when topped off down to a half tank. From a half tank on down to E, it goes really fast.

All of the above recommendations are spot on if you really are getting such terrible mileage. Just don't base your mpg's on a half tank.