[Roger Bolick]

There is a huge amount of information on the Internet... and the more you read the more confusing it becomes. What follows is a summary.

Many of those presenting "factual" information are SELLING high performane pumps or regulators... its to their advantage to be conservative... recall the muscle cars of the 60's ran 3/8" lines with the HP engines, no return lines, no engine compartment regulator... just a mechanical pump. Of course most street cars will NEVER see continuous usage at max fuel draw, just bursts of action... so guess it also depends on usage, street terror or NASCAR.

To prove this data is REALLY CONSERVATIVE, look at a Ford 190 liter/hr (50 GPH) in tank fuel pump. Built with 3/8" output and 1/4" return line, this violates the recommended sizes below...

Seems the feed line size is related to the HP (*Mallory site has original data):
Up to 500HP 3/8 -6AN

Pump size GPH is either .23 x HP for a dead head or .17 x HP with a return.
500HP x .23 = 115 GPH fuel pump dead head
500HP x .17 = 85 GPH pump with regulator

Return line is dependent on Pump size as worse case is idle and you need to be able to easily bleed off enough fuel to allow the regulator to work. At WOT there is little fuel returned to the tank, thus the return size is unimportant.

Up to 90 GPH 3/8 -6AN

I'd guess the one size smaller return line is used on production cars mostly to distinguish it from the fuel line, i.e. idiot proof!

Seems your 3/8" fuel line would have a 3/8" return line (seems normal to use one size smaller if the pump isn't oversized, guess the assumption is some percentage of the flow is used even at idle and thus the return doesn't present a restriction). I see no reason to ever make it larger, though it wouldn't hurt anything.

One important item is to ease the feed to the pump, if you look at the system flow numbers running the next larger size (in this case -8An or 1/2") line from the tank to the pump is an excellent idea. Then use the 3/8" or -6AN for the longer main line and a 3/8" (or 5/16") line for the return.

A professionally modified Kirkham tank I owned had the original bung bumped up a size with a 1/2" pickup tube and a second large bung installed for the return line.