Depends on the API gravity of the gasoline, which varies by grade and
refinery.  Usually, regular unleaded gasoline has a gravity of around
58 and a weight per gallon of 6.216 pounds per gallon.  Premium
gasoline may have a gravity of 54, or 6.350 pounds per gallon.

and yes the door sticker and calc. oh, look on your tires sidewall for
load psi specs.

or look on the sticker for towing specs and get a trailer, but
shouldn't be a problem hauling that weight in you car.

I need to transport 560 lbs. of cement in my 2000 Outback. Is it OK to
load up the back with that much and keep it loaded for 24 hours until
I get to the destination?  

I'm guessing it's OK becaus it's like having 3 good-sized adults in
the car, but I just want to be sure.