So i have a 94 straight six, and when i bought it i was informed that the front gas tank leaked, so i started using the back, it doesnt leak unless it is filled up and then i overflows from top of gas tank, and besides all that i have beem gettimg 6 mpg , i understand with a lift mpg goes down, but 6 is a little to low, let me know wat u think is the problem?

 

Just sounds like yer seal on the top of the rear tank is bad, easy fix just a headache to do it. As far as the terrible mpgs I'd check yer fuel pressure first off. Not positive what it should be on that but I'm thinking around 38 to 40 psi. If thats all good I'd say you just need a good ol tune up. Plugs, wires, cap, rotor. Wouldnt hurt to check for codes on the computer either.

 

Fuel pressure on a 4.9 should be 50-60 psi. I agree with farmer12 that the fuel leak is probably the pump seal but iI would check out the vent check valve, filler tube and tank it self for a hole also.