Pump shock

OK today I took my tiny van to the gas pump...she's supposed to get a minimum of 22 mpg and a max of 30. Well, she only got 20. I am thinking I should check her tires again. It seems that when her tire pressure goes down, so does her gas mileage. I love driving her, she's fun to drive...but she can be a guzzler when her tire pressure is low.