Well, most commonly the only thing to blame is the Right Foot!
Have been using a 2007 prosmatic since new, with 215/60R15 Yoko DB tyres on original rims. The average around town has been approx 9 to 9.5 in summer with AC, and 9.5-10km/l in winter without AC. Average driving is gentle, with occaisonal flooring of throttle. On motorway M2, trips to lahore between 120-130kmph gives 13-14km/l with ac on.
(Engine oil used is mostly Mobil-1 5-50, changing after 7500km)