I have just recently purchased a low mileage 2015 Suzuki Grand Vitara Sport 5 speed manual and have been curious about the reported strange behaviour of the fuel gauge so watched it closely from full to the light coming on recently. This involved mainly city driving with about 170 km driving on secondary country roads. The gauge seemed to go down in a reasonably linear fashion giving about 130 km per quarter tank with the light coming on at 520 km and the gauge just above empty. The computer indicated 9.3 l/100km but the tank took about 51.5 litres to fill giving an actual usage of 9.9 l/100km.
The tank is still 1/4 full when the light comes on so there is still about 150km reserve at this stage. The car is designed to be used in remote areas so this has likely been done on purpose to reduce the possibility of running out of fuel in the middle of no-where!