According to my experience and knowledge you can increase ur fuel efficiency by 1-2 km/l by focucing on following things
1. tires
use the correct tire size. and air level .good road tires will increase the fuel consumption
2. Petral level on the tank
try to keep more than 1/2 full
3. Gear box
check the oil level in gear box . change gear oil abt every 25000kms
4. Engine oil .
use a qulity engine oil and check the oil levels and change the oil abt every 3000-4000 kms
4 . breaks
check the break system and ensure there is no break blinds.
6. belts. and air filter
check the condition of belts, air filter and replace if need ( fan, AC and power pump belt) .
7. AC gas level.
keep the correct AC gas level in high side
8. silencer.
check the silencer for leaks and dont use beat silencers. silencers with leaks will give the wrong figures with the oxygen sensor
9. throttle body
all the sensors must work 100% for good pickup and good fuel consumption. specially ideal up sensor.
9. body kits
dont use the large body kits for the vehicle its increase the air resist on. body kits will reduce the consumption in long rides.