Im switching from a 225 to a 318, is there a big difference in fuel cost?

I am thinking about buying a 73 Dart that has a 318 engine in it. The only thing I am worried about is I have been driving my 75 Duster that has a 225 in it for the last year and it really has been good on gas. I dont want to switch to the 318 if its going to cost me a bundle every time I have to fill up. Does anyone know if there is going to be a huge difference in fuel price? I dont want to buy the Dart and feel like I cant use it as my daily driver because of how much money it will take to keep it moving.