Here is something interesting I just noted.
Early P-38's had 300US Gal. J-10 and later blocks had 420US Gal, but were only fitted with two 55gal tanks. 2x55 = 110. 110+300=410.
I have an original pilots manual somewhere around here. In it it talks about something very interesting. The differance between normal fuel load, and USEABLE fuel load. Say you have a 20 gal tank on your car. You cant use ALL 20 gallons! Only about 19 is useable till the fuel pump cant suck any more. Aircraft were more so, and some fuel tanks had as many as 5 gallons unuseable. I can only guess that the wing tip tanks are 60 gal each, with 55 gal useable. Again, I need to dig through and find this information. Can anyone confirm this?