When i first got my truck about 6 months ago it got awsome milage like 400 kms off the front tank but it really needed a tune up cuz it had not been tuned up in about 3 years (loggers truck) and ever since i did the tune up and put true dual exhaust on my milage has dropped to about 250 kms off the front tank any ideas on what is causing it?

1992 F250 r/c l/b 4X4 351 5spd

 

How's the rear tank?

Maybe you lost to much back pressure after putting on the true duel's so you're running a bit richer.

 

I'd double check anything that was changed during the tuneup. Check the plugs to make sure they're gapped correctly. Also, you may want to clear the computer by disconnecting the battery for 15 minutes so the computer can relearn the engine. If it hadn't been tuned in 3 years and you added dual exhaust then the fuel curve may have changed.

 

so should i take off the glasspacks and put some chambered mufflers on?
1992 F250 r/c l/b 4X4 351 5spd

 

im not sure about the 351 but the 302 has very little backpresure to begin with
before putting duals on my truck a checked with a few shops to see if it woudl cause a problem with backpresure and they all said the same thing
mine was very hard on fuel right after the duals were put on and the exhaust stunk real bad
ran very rich
i reset the computer and it ran great after
milage is back to normal

 

 

Check your O2 sensor(s). If they got disconnected or damaged during the exhaust install, the engine is going to be running dead rich, drinking gas like crazy and be way down on power. Your plugs should tell the tale.