Best two mods in my Cordoba has been:
175/80R14 Toyo 350 tires to front. Any tires with same dimension will do the trick.
And maybe 175/65R14 tires to rear or even 165/70R13 to rear. I think the last ones worked better for fuel economy, even if the 175/65R14 are michelin energy savers...
Other one reguires no mods. Use push and glide speeds under 95 km/h if your car is stock.
If you have lots of mods like my car you can continue doing that at higher speeds also.
Just today drived behind a bus @ 100 km/h with consumpion of 2.2l/100km for 40 kilometers
.
I would buy the ligthtest car possible with a TDI engine.