what can be done to improve economy on a 98 GMC truck? currently getting 12. 5.7l Vortec V-8 350.

 

What for motor? More information please..

Do away w/ clutch fan, run electrical fans..
Cold air intake are just two that pop off the head.
Different tires.

 

freer flowing exhaust would help, CIA, and a shift kit would help.

 

Another thing peculiar to the Vortec engines in those was the Injection system.

Get it on a Tech2 and see if the Injectors are all firing correctly. Those have been known to fail prematurely, giving cylinder misfire and poor economy.

If it is a half ton even with 4x4 unless it is lifted, I would say you should see 16-18 Hwy easily.

 

I put a K&N intake, headers, synthetic fluids, cat back Gibson system, and Hypertech chip on mine. Got 16 MPG and 330,000 miles before the rebuild. It was a 2500, 4x4, lb, and auto.