My Honda City

Petrol: Shell Super
Location: Penang Road
Liter Filled: 36.97 liter
Distance Traveled: 621.9 km
Onboard Computer KM/Liter: 15.5 km/liter
Manually Computed KM/Liter: 16.8 km/liter
City Driving: 40%
Hwy Driving: 60%

Again, the KM/Liter differs between the onboard computer and manual computation. Using back the old computing method, the onboard computer is quite close to the manual computation method. Now the question is, which computation method should I continue to use?