What quantity did you put? It is 3.2 L for 1000cc Vitz. This is just a check; has got nothing to do with mileage.
To gain more mileage, check the tire inflation pressure. It should be 30/32 psi as per manual. Also get tire alignment and camber verified. This is really a fuel eater.
Stuttering usually happens with 1000cc vitz, primarily due to low octane number of fuel available in Pakistan and also when AC switch / other electronics is already turned ON prior to engine starting. Does your car start in half 'sulf', or does it go 'chuu chuu chuuu chuu huurrrrrr' and then starts?