what's the best swap for the 2.9 pos and still get good gas mileage

 

I have been reading about the a turbo diesel for a Ranger:
Ranger / 1.9L TDI Conversion - TD conversions - Switch from Gas to Diesel

all it takes is money.

 

Welcome to the forum.

Older V6 engines, even fuel injected, will not be great, 17mpg if you are careful, but more likely less.

4cyl gas can get you above 22mpg

4cyl diesel can push 30mpg, diesel fuel has more energy per gallon, so always better MPG, but diesel engines are also more efficient than gasoline of the same years
Gas engine are about 25% efficient, newer model are getting closer to 30%
Diesels are 30%, newer models over 35%

Pickup trucks, even lighter ones are not aerodynamic so not the best choice if MPG is your goal.

The 2.9l in good running condition with manual trans can get over 20MPG on long highway trips, seen posts claiming 25MPG.
If you are under 17MPG then there could be something wrong