alright dude so u have a heavy car, i drive the 06 n well a 1KRFE engine is supposed to give u 16km/l in city and 20km/l on long but thats on japanese roads with the fuel that they use there, unfortunately the fuel that is sold here as hi-octane, im not even sure that THAT wud match the fuel grade they maintain in japan coz my car wud always keep knocking even with the best fuel avaiable.
ive therefore converted to CNG and the car is running fine, except that on petrol its missing at idle rpm,the simple solution to that is reset the computer of the car, take the -ve terminal of the battery off and wait for abt 3 to 4 mins, reconnect -ve terminal of the battery and make sure before doing this the car was converted to petrol and switched off. Then start it and run it on petrol, it will run fine till u change it on cng again.
If u wana get best from ur car on petrol then take the following steps
1)Full throttle body cleaning with proper fluid.
2)Use original air filter, comes around 900 rs
3)Use full synthetic oil, mobile 1 or ZIC A+
Change ur oil filter when half running of the oil is done, i.e if u put mobil1, it will give u abt 10000km but thats if u change the oil filter at 5000 km, otherwise it will only give u abt 7000 km total, this is because of dusty conditions in pakistan.
4)Get brand new spark plugs for ur car they will cost 900 to 1100 each so abt 3300 for a set of 3. BUT REMEMBER, now this is THE MOST important thing abt these new plugs, the plugs used in this car are the longer ones, this is a newer type and they are irridium tipped, IF THE GAP of these plugs is even changed by a 0.01mm, consider them ruined, our CNG tuners usually tap them to make the gap smaller, well my friend that works in older cars, on these engines like corolla X and vitz if the plug gap is changed the car will vibrate like mad on idle rpm and whenever car is in P or whenever a brake is applied and car is abt to come to a stop. THE GAP ON THSE PLUGS CANNOT BE DISTURBBED. Just keep em clean and thats it.
5) Now the fuel filter, whenever u wana get it changed, remember, just the filter part which costs abt 4000, changing only that will not help coz the filter in this model is contained in the whole pump and filter assembly. change that whole assembly along with the metal part which is the pump, THAT WHOLE costs abt 14000 rs, only then u will notice a change in the performance.
6) If ur car has stock tyres then ur consumption will improve but if u have alloys and fat tyres, something like 195s, that wud reduce ur average so keep thinner tyres with a better tread design for control.