There are many factors which affect fuel efficiency. If there is no missing in your car, plugs are working fine.
To be sure, do this simple check. Start your car, open the bonnet, one by one disconnect plug cables. You will engine vibrating or doing missing when cable is removed. Connect the cable and this missing / vibration will end. This step will ensure that the specific plug is working properly.
Now the other factors.
Quality air filter: I have driven 660cc alto for 25km in a litre with good air filter element and 16 km in a litre with cheap bad quality element.
I have experimented with all local brands but found Leppon air filter to be a decent quality. It’s for Rs 450.
Air pressure in tyres: Go for the tyre pressure written on inside of driver door. Most people fill the tyres with 28psi whereas manual says 30 or 32 psi.
Tyre size: the bigger the tyres the lower will be mileage. Always stick to stock size as near as possible.
Quality fuel