Leaky Fuel Tank

Howdy boys and girls!

When I bought my truck, I was told it had a brand new gas tank on it, and I looked at it and yeah it looks pretty brand spanking new.. The problem is that when I fill it to the top, it leaks out the top - I'm not sure if its overflowing (well I guess it is lol) or if its an actual leak. Is there a gasket or something on the top of the tank that could have gone that would cause this or is it the tank? I suppose I could drop the tank and find out but I'm trying to delay that one as much as possible! It's a 1990 S15 Jimmy 4x4 with the 4.3 in it.

A side question - would this leaky tank contribute to bad fuel economy?

Thanks In Advance
Ben

Could be the sending unit gasket, or it could be the filler neck isn't tight enough, or even the vents could not be installed properly. I would If you can't readily see something and it's flowing off the top of the tank in the middle, most likely the sending unit gasket (or even a bad hose) Pull the tank and inspect it.