I found this online, no real reference unfortunately but it's interesting regardless.
According the the WWII Training Manual for the P-40 (written primarily for the later models) :
"The fuselage tank is called the vented tank, which means there is an overflow line between this tank and the carburetor. The fuel line carries excess gas from the carburetor back to the fuselage tank. At high engine rpm, the fuel overflow may run as high as 10 or 15 gallons per hour.
Always use the fuselage tank for takeoff. During takeoff the chance of fuel overflow is especially high, and unless you are using the fuselage tank--the only one built to handle overflow gas--the overflow is lost.
After takeoff, always use at least 15 gallons from the fuselage tank before switching to another fuel tank.
The fuselage tank accumulates possible overflows from the other tanks during flight. Always turn the selector handle to the fuselage tank if it appears you have run out of gas. The fuselage tank probably contains a few gallons of overflow fuel."
The training manual later suggests: "Climb at a speed of 150-160 mph. When you reach 1000 feet, switch from the fuselage tank to the belly tank and fly on the belly tank until it is empty." I believe one of the considerations for doing this is that the belly tank does not have a fuel gage, so it is best to use this fuel first and have a known quantity in the other tanks.
