Driving this specific Honda city generation since 2009 and I can carefully say don't believe what on board mileage calculator on longer routes.
For local driving it works fine with less variation.
One thing I've noticed is that pre facelift models with red odo cluster have more variation and the facelift models have very less variation in actual mileage and what on board calculator shows.
I get around 15-15.5 km/l locally and on board calculator shows 16.0km/l.
Just my 2 cents.
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