I'm Afrersize: My most valued cars and modifications
 

I do not own a car yet though I will next year, the below hatchbacks are the vehicles that I believe according to my research are most practical, reliable, and efficient.

The modifications are only the big systems and accesories that I believe would cause great improvement in fuel efficiency. I know of plenty of other mods that would also be necessary in making an ultimate FE, practical car as listed on this site as well.
CAR
07-08 Toyota Yaris 5-door
Ford Aspire four-door
95-98 Geo/Chevy Metro 2-door hatch
99-03 Volkswagen Golf TDI
94-95 Honda Civic vx
-06 Honda Insight
MODIFICATIONS
-Hydrogen Dry Cell system (two dry cells should go in Golf)
-SVO (Straight Vegetable Oil) system, only on the Golf
-low resistance tires that also make less noise would be great (though I dont know if they exist)
- also the aerodynamics should be also applied such as wheel covers, grill cover, removable kammback etc.

My home made Hydrogen Dry cell
 

The plates on my Dry cell are made of aluminum therefore they dont conduct much eletricity compared to stainless steel, which is what ratailers of Hydrogen Dry cell systems use. Since less electricity is conducted, not much hydrogen is produced, this would result in a very small efficiency gain if any. I don't know about its efficiency since I havent put it on to anything.

Welcome to the site.

Any of those cars would give you some very decent mileage. It'll probably come down to what is available in your area and how much you want to spend.

As for the mods, I'd forget about the hydrogen. You won't find any gains there.

I'm not certain of this, but I don't think the newer TDIs are happy running SVO.

LRR tires and aero mods are the bread and butter of mods.

Don't forget driving technique! You'll by and far see your largest gains from driving efficiently.

I read an Article
 

it said that older diesels had these rubber seals that were corroded by SVO, it said newer diesels worked fine on SVO.

Newer diesels have injectors with much smaller orifices which plug up much easier. This is a problem with SVO due to particles in the oil and higher viscosity in cold weather.

I am no expert on this, I don't own a diesel nor have I looked with much depth into converting to SVO. I have read about these issues on the forum. I would suggest doing more reading on the topic though.