Into the weeds engine design for fuel efficiency discussion.

Where is the dividing line between total build for max efficiency or just freshen up a good combo and pay a bit more in gas?

Adding a Gear Vendors OD will set you back about $2500. How long will the better gas mileage take to break even of the unit costs and having the driveshaft shortened?

I mean, take my 78 LRT. If I needed it to tow a small trailer a lot then I'd maybe put a complete 2000's two wheel drive Dodge 1500's wiring, 5.9 Magnum engine and A518 trans in it then tweek it with a "HP Tuner".