Hello guys, new member here. I recently got a 1969 Ford Mustang coupe with a 200 inline 6 engine and 3 speed manual transmission. It's in great condition, has had regular maintenance and has just recently had a tuneup/oilchange. However, everything is stock on it with no modifications and currently I am getting around 12-18mpg with a mix of city and highway driving.
I would really like to make this my daily driver now and am prepared to invest money to make this happen. What can I do to improve gas mileage and make this more practical to drive daily? I would prefer to do this without swapping out a new engine or transmission.