This is the absolute number one as it directly relates to producing force needed to accelerrate mass.
Friction issues are secondary but also very very important.
Starting with good, proper octane grade, fuel in the fuel tank preferably with some lubricant added such as Marvel Mystery Oil, all below can and do significantly affect fuel economy:
1. Fuel lines that are not rusted, fuel filter that is not clogged (change either). 2. Fuel injectors atomizing fuel properly for better, more efficient fuel burn (injector cleaning, or replacement). 3. Good spark plugs make a big difference, especially if worn out (change frequently as TOP (!) performance degrades quickly with most spark plugs). 3. Engine oil of light weight, preferably synthetic for reduced engine friction, likewise change regularly. 4. EGR and exhaust system working properly so excessive obstruction does not exists here. 5. Similarly air delivery system, properly working Idle Air control valve, clean air filter is a must (change frequently, especially in dusty areas). 6. Synthetic oil of proper weight, preferably on the light side, in all manual/auto transmissions, and particularly differentials. 7. Tires properly inflated (slight bias for overinflating). 8. Windows rolled up completely to reduce any drag. 9. Car surface clean, heavily waxed and well polished to reduce surface friction (especially helpful on long distance higher speed trips).
All the above plus proper driving technique will help reduce fuel consumption significantly. One not recommended practice, and in fact illegal in many places, is coasting out of gear, that is in neutral. This can help save some fuel but is very, I repeat, very dangerous, and not cost effective as much more wear is put on the brakes.
Happy Subaru motoring everyone,