[Formulajones]

It depends on how far you are regulating it down, how big the pump is that is required to feed the HP you have etc...

If you are running a simple 500hp setup with a carb, and regulating it down to 6-7 psi, you can run a typical 255 LPH pump. In which case you can probably get away with a 3/8 return line. I still generally run a 1/2" return anyway because the cost is the same, and it leaves room to grow later. I tend to like running a bigger pump than required anyway to give some wiggle room that the formulas don't take into account like friction and G forces that affect fuel delivery. Plus most of the pump ratings are free flow anyway.