Thank you so much for explanation brothers.
What I have concluded after literature review on internet is: High octane should be given to the rides who have knocking engines. Without knocking, high octane is just a waste of money. All engines are designed by keeping in mind a "RON-rating".
What are octane ratings?
Octane ratings measure a gasoline's ability to resist engine knock — a rattling or pinging sound that results from premature ignition of the compressed fuel-air mixture in one or more cylinders. Most gas stations offer three octane grades: regular (usually 87 octane), mid-grade (usually 89 octane), and premium (usually 92 or 93).
In Pakistan the normal grade is 87. I am assuming this because most bids on PSO website give number mogas - RON - 87 to their petrol.
link for PSO: http://www.psopk.com/suppliers/tender_detail.php?tid=1666
link for shell: ITB MOGAS-87 RON MIN (UNLEADAD)
The RON rating for toyota 1KR-FE engine (Toyota vitz 1.0) is 95! That is why it knocks a lot on normal fuel (87)
More powerful engines tolerate low octane fuels. Less powerful engines really mind with less octane numbers if they are designed by keeping in mine higher RON rating. So 1kr-fe reacts more to low octane fuel than 2SZ-fe and 2NZ-fe engines.
Final note: Hi-octane does not increase mileage. May be it can for few engines. But it is not made for increasing mileage. It might also not help in a "cleaned" engine i.e. it is not necessary that you get less carbon deposits with high-octane. It might also not give you more power! I have seen people putting high octane and getting low mileage than normal octane. It just is mixed with anti-knocking compounds. So I recommend that if your engine knocks, go for high octanes. Other wise pasay bachao bhaio!
*This is just a literature review. It may prove wrong. I myself has no expertise over this