Worste tank ever!

[Zerocide]

So it started last week when i filled my truck up when it was on empty, it only filled up to 15 gallons when i got a 17 gallon tank. Thought it was a little weird, espically when i turned the car on to find out my tank was only reading 4/5 a tank. Passed it off as a fluke thinking maybe by gauge was broke. so a week later on today my tank reads 1/5 full and only at 128 miles, im not worried because i had already figured my gauge was broken, but just to double check i go to fill up the truck anyway to prove to myself its broke. well this time the tank fills up at 15 gallons when it had a fifth of the tank left, odd... then i remembered i filled the truck up with 15 last week and im only at 128, thats only 8.5 mpg! Now i use E-85 and expect to get around 13-14 mpg but 8.5 is the worse ive ever heard of, now im gona assume that it was a bad tank of gas considering it said it filled up to 15 then the gauge said it was only 4/5.