Ok i just did the headgaskets on my truck and while i had it all apart i did a complete tune up cap rotor plus and wires and since my exhaust broke i put long tube headers on. Well at first before i got the full exhaust on i was getting about 150 miles to my front tank, then i got my exhaust, 2 1/4 all the way to the back and a muffler but no o2 sensor yet and now I'm getting less than 120 to the tank.
Next issue is I'm still losing coolant. Doesn't drip while sitting and no smoke. I can't fugue it out. Any where else to look? New water pump when i bought the truck and t stat.

 

get the O2 sensor fixed....

I get 400 miles to my tank...city driving

stay out of the throttle so much

 

I drive 1 mile to work every day through downtown so not much throttle used there. What size gas tank do you have? Mine is the 13 gal front tank on my ext cab short box

 

I'm the same way, mines a 1990, but I haven't done anything other than a tune up, removed the cat, and just done oil changes and other maintence
She really likes to go through the gas

 

Mine is a 302 forgot to mention

 

X2!

 

That's pretty bad...I'm getting around 400 miles between 2 tanks with my 302 supercab. The coolant thing would worry me the most...especially after doing head gaskets. Checked the oil recently?

 

Yea oil is good. Coolant just a ghost leak. Maybe next week I'm gonna get my coolant system pressure tested

 

After doing the work, did you completely bleed all the air out of the system including the heater? Could have been an air pocket that made it's way back to the radiator?

For other issued I would run the self diagnostic and retrieve codes.

 

The O2 sensor is used to adjust fuel air mixture so driving without it could do some harm to the engine along with bad gas mileage. Change the radiator cap and check the line that goes from the radiator to the recovery tank.
YMMV
rikard