[Old Henry]

Since the mechanical pump produces over 6 times the volume necessary to sustain 75 mph you can return 5/6 of the fuel by the return line and still have enough flow into the carb. As you said, your return line that's smaller than your supply line will create sufficient pressure and flow without any further restrictions. I figured that all out when I was about to install a return line but then figured out that it won't help with my most common vapor lock problem - starting the engine after sitting hot out in the sun shut off for 10-20 minutes. Oh, it starts OK because all of the fuel hasn't boiled out of the carburetor but since the gas has all boiled out of the pump after going a few feet the engine uses up the gas in the carb, hasn't got any more from the pump, and dies and won't start again. I don't see a return line helping with that problem. It will be interesting to see if it does on yours.