gas tank evaporation and MPG

[sqrlnts]

I calculated my MPG for my last tank and came up with 8.5 MPG!

Truck has a 350 2bbl carb, Th350 trans, and 3.73 gears. All in city driving no hot rodding around, no real high freeway speeds.

I have noticed that I can see a stain developing on the top of my gas tank (externally mounted '73, 25gal tank). I think the stain is from a very small leak on the sending unit vent tube. In the hot ash Arizona sun how much fuel could I be losing? To be fair I only noticed the small pool of gas around the tube connection tonight when i took the photo. Normally it only showed as the black stain you see, no real leak or drip of gas on the floor.

The gas cap seals fine and I have the return port plugged cause I don't have one. Could this be some of my poor MPG woes?

I just find it hard to believe that a 2bbl 350 can drink this much gas