I've owned two Aquas (2014 and 2016). They are very economical but don't expect wonders. There is no substitute to driving with a light foot when fuel economy/ conservation is "the only priority". Its simple equation. On highways/ motorways 130km/hr to 150 km/hr will definitely entail more fuel expenditure irrespective of your engine tuning. At the most, i used to get 21/22 kms/liter as my fuel average with AC and my vehicle was in pristine condition (low mileage, newer plugs, filters, engine oil, CVT gear oil etc) and sane driving. On highways it came down to 17/18 km/liter. I would suggest following:
1. Make sure the engine oil is 0W-20 as more viscous engine oil "reduces" fuel economy
2. Air filter is genuine and clean
3. Service your batteries
4. Full diagnostic check is carried out.