Easily get? Perodua famous for overstating by multiple times their FC. New Alza for instance claim 22 km/l, even with purely highway drive and no jam from KL to JB I was only hitting 14 km/l. Fuelly.com shows the old Axia (the best FC Perodua car so far) roughly 15 km/l with mixed driving and 17+ km/l for highway driving, no way the new one easily gets 20 km/l.

This 'hardcore one' is still below 19 km/l, and like anything else on fuelly you need to look at the data distribution for a good idea of how the real FC is. Can clearly see two peaks as I have mentioned, one at 15.5/16 and one at 17.5. Most cars have this pattern, where the lower peak corresponds to more mixed mode driving (city + highway) and the higher peak corresponds to primarily highway driving. Compare, for example, the Viva which has the same pattern with peaks at 13.5 km/l and roughly 16 km/l.

The reason to look at the distribution is because, as you said, 'was this real?'. Single outlier results, chance to be fake (or at least misttyped) is higher, that's why we look at the averages.

'Too early to assume'

'We can assume'

A bit confusing right? Would be really good if they can get that improvement. But again must be apples to apples comparison (i.e. my new Alza has much better FC than my old Alza... obviously so just from the fact the old Alza was almost 14 years old at that point, so comparing 1 year old and 14 year old car) of similar age vehicles with similar usage.

My experience with the Perodua FC calculation is it is consistently 'off' from my actual calculated fuel consumption. For me I don't calculate FC based on individual pump up (because that would depend on how full you fill up in the initial pump and 2nd pump both), instead I use an app to track mileage and amount over time. Then I track not the per-fueling stats but the average stats over time (you can also running average to see trends if your driving varies by time of year). By this calculation the dash estimate is always too optimistic in my new Alza, by about 1 km/l. The possible source of error here would be how accurately the internal sensor can measure the exact amount of fuel each time you pump (definitely much less accurate than the petrol pump measurement due to profit motive of petrol distributor compared to car manufacturer). So imagine car owners A and B with exact same vehicle, but A always fill 'full tank' which is slightly more than B 'full tank', doubt that the car can differentiate the total amount of fuel in that case due to resolution/sensitivity of sensor.

tl;dr - reading from dashboard give general idea, but not as reliable. Measure yourself.