They don't use tapered NPT (National Pipe Thread) fittings for fuel lines. The fuel line itself is 3/8", but the fitting that is coming out of that fuel pump in the picture above is an inverted flare (SAE J512). That flared junction is the sealing surface, unlike a NPT tapered thread which uses that taper to seal. Don't use Teflon tape or thread sealant on an inverted flare fitting either, it's pointless.
The thread pitch for a 3/8 inverted flare fitting is: 5/8-18, which is a straight thread pitch.
Also, for fuel and brake lines, you're going to want a double flare on the tubing end as well.
Scroll down to 'Inverted Flare' in the links below for this conversation, but a lot of good reference info in general.
https://www.titanfittings.com/us-thread-chart_a/342.htm
https://www.hydraulicsdirect.com/Fitting-Thread-Chart-s/1934.htm
Found this chart too: