The truck is a 1988 F250 with the 7.3l idi, it had the 5/16 fuel return rail on it. I did not realize it had said size line and purchased a new kit to replace all of the old cracked and leaking lines. I opened the kit and began replacing them only to realize the vast size difference between the two. The new kit is a 3/16. Which I believe was stock.
So my questions are;
Will the smaller return line mess with the way the truck runs?
Is there a horspower cap limit for the smaller lines?
What are the benifits of running the bigger line?

The truck is going to be my daily driver and I have just finished rebuilding the motor and I am not looking to build a monster power truck. I am trying to get the most fuel economy as possible, while still having enough power to tow a horse trailer on occasion. Any help any of you can offer will be greatly appreciated.

 

Sounds like you got the kit for the 6.9, I would return it and get the proper sized kit. I think you could probably use the smaller one, but there is a reason ford upgraded to the bigger lines on the 7.3.