What should be the gas mileage for a 48 1 ton with single wheels and with everything stock(right down to the tires

)?When I got it I thought it should get around 16 mpg at 45 mph.But after takeing it for a spin around the block at 45 it's running the motor fast and now I'm thinking maybe it isn't going to be as good as I thought.Not that it matters much at this point, almost have it ready to start driveing,but just thought I would ask since I couldn't find anything in the search.