I’ll start: put a tall gear in every transmission.
My present car has a 5.7 liter engine, yet in sixth gear (.50:1 ratio) I can get close to 30 MPG on the highway. Also, I noodle along at 1,900 RPM at 70 MPH - thus reducing wear and tear on the engine.
By contrast, a previous car I had with a 5.0 liter engine and a five speed never made 30 MPG and turned nearly 3,000 RPM at 70.
Why do so few cars have a tall overdrive gear? Cost is hardly an issue at all.