The Toyota RunX is the ‘Sprinter’ version of the Toyota Corolla Allex, like the Toyota IST, these three Toyota vehicle models are hatch backs.
They all have excellent aerodynamics and handling attributes while their design and 16 valve DOHC Electronic Fuel Injection engines enhance fuel economy. The 1.3 Litre Toyota IST offers slightly better fuel economy (19.6 KM/L) on the highway than the 1.5 litre Run X (18KM/L).
However, the 1.3 litre IST will be sluggish (86 Horse power @ 4,400rpm) driving uphill especially when carrying four adults. The 1.5 litre RunX will offer a better balance between engine power and fuel economy (104 Horse power @ 6,000rpm) when carrying four adults and driving uphill.
It should be noted that a smaller engine will not give you the expected fuel economy result when you overload it.
The Toyota IST fuel economy result may drop down below that of the RunX if you carry four adults and luggage because the engine revolutions (and fuel consumption) will keep high as it struggles to balance the vehicle load and the small engine power.
You can improve the fuel economy of the Toyota RunX by using the Overdrive facility when driving on the highway, driving less aggressively, not overloading it and using Shell fuel Save Unleaded which has additives which clean and lubricate the engine to improve its economy and power.