some of that (the highway rating especially) is gearing, the final drive is shorter and the 6th gear in the ZF manual is not nearly as tall as the 8th in the 8H

some of that (the highway rating especially) is gearing, the final drive is shorter and the 6th gear in the ZF manual is not nearly as tall as the 8th in the 8HP. This is a common issue in auto vs manual MPG comparisons these days