Aluminum Slant 6 engine

In my '64 Dart 170, with the stock 170cuin/3spd manual, 3.23 gear, I'd average 23mpg. When I went to a 4spd OD and 3.55 gear I was averaging 27mpg. Most of my driving is in town too.

Now with a 360/4spd 3.55 I average about 12mpg.