sir, ang main difference nilang dalawa is 4X4 ang GLS,most suitable for off-roading, while yung GLX is only 4X2, more suitable on city or highway driving. they both use manual tranny.
mitsubishi stradas here only use diesel engines. i don't know sa ibang countries if they offer strada using other fuels.
sa fuel economy naman sir, generally matipid sa diesel ang strada. mine is about 8.2-9km/L right now, pero nung bago ako magupgrade ng gulong umaabot ng 10km/L ang consumption ng strada namin. im not sure pero i think other strada owners here even have fuel consumptions of up to 12km/L, so i can say na matipid sa krudo ang strada.
hope this helps sir.
