Vios should lead the economy contest, being lower and shorter than the Avanza better engine technology than the Sentra, and lower weight than both. The Sentra, I think, should come second being lighter and more aerodynamic than the Avanza. I suspect the Avanza 1.3 will head the Lite Ace way, underpowered and undertorqued for a 7-passenger vehicle that would result to slow performance and/or not-so-good fuel mileage.
My friend's Vios 1.3 did as much as 17 km/liter, but economy runs should yield even better results.
No, RWD doesn't necessarily make a difference in economy. Compared to FWD, it's a bit heavier with the driveshaft in place, but the weight is better distributed. 4WD is another story though.
former sentra gx(m/t) owner. fuel consumption is 14-15 km/l highway driving with conservative driving (80-100km/h). Medyo malakas sa gas pag city driving( 7-8km/l) mabigat kasi body.