Hey everyone, im looking at getting a new truck probably an early 90's like 89-91 F-350 crew LB. WHat fuel economy difference would i expect between a 5.8 or 7.5 and what am i lookin at highway wise for both. thx for any info

You are looking at some real low MPG with either choice!

5.8 = 7-10 MPG
7.5 = less

I have a 92 F350 Crewcab long bed that has seen 17.5 highway. Usually averages 15-16. Considering the size of the thing, it's not too bad really. I have the 5-speed manual, 4.10 L/S, a shell, and it's riding on stock wheels with stock size tires.