15.0 mpg to 21.6 mpg!

I hate to be a doubting Thomas, but I would put more faith in your MPG figures if you had used at least a full tank of gas (200 -300 mile trip) during both runs on a highway at a steady speed. On the other hand, I hope you are right. I just returned from a 4,600 mile road trip in my '72 318 two-barrel automatic BBody with 2.71 rear end and averaged 16-18 MPG. So 21.6 MPG in a 360 I think is phenomenal.