Everyweek, I put about 40-42 litres of petrol and lasts me for 350-400Kms, so it's close to 10-11litres per 100Kms. I have never run the car down to the red light to get a more accurate fuel economy from a FULL tank.
However, the backfire is only occassional like once a week and normally happens when I shift from 2nd to 3rd around 4500 rpms.
Also, about 2months ago, I had full injector service done. But is used to backfire before that as well, so i don't think I can blame the mechanic.