Mileage Question

Hi new to this forum. I recently started driving my slant 6 Duster around Tampa because of college and am getting mileage of around 12mpg in town. I was wondering if this is normal because on the interstate riding about 70-75mph I get 21mpg. (Running with a one barrel and an automatic transmisson)
What could I do to get better mileage?
Thanks.
Mopar or no car! :toothy10: