What is your mileage? If you're filling it at a quarter tank, then you'll be putting less in. Not sure if that's how you're figuring it.
If that is an absolutely empty tank and only getting 270 miles I'd check your e-brake and driving style. Even if I drive faster during the week, I'm still getting 28 mpg.

 

I have 325 miles since I last filled up and I am sitting at a little above 1/4 tank, going by previous fill ups at 1/4 is about 10 gallons so that is still over 32 MPG.

 

My MPG has been around 24-25, mostly highway.

 

Are you running E85 in it?

 

Nope, regular unleaded.