Just went to test it, and there were 3 contradicting parts.
Test: All Spits/Seafire have a reported Slipper tank of 30 gallons in the hangar.
Seafire: Report as 37 Gallons in the Secondary Weapon select, and when dropped loses 36 Gallons of Fuel.
Spitfire Mrk IX: Reports as the same.
Spitfire Mrk XIII: Reports as the same.
I stopped there, but would guess the Mrk XIV and Mrk XVI report the same.
Test Pics:
In the Hangar SeaFire:
Showing The 37 gallons in Secondary Mrk IX: (Same as Seafire and Mrk XIII)
Showing after the drop Mrk IX: 185 Gallons - 149 = 36 (138-102 for SeaFire and Spit Mrk IX)
Hope this helps some.