zainrock5: The Vitz (1999-2004) engine: 1SZ-FE, 997cc, does about 13 km/litre with AC on 100% of the time, in stop & go city driving. Over longer drives at an average of 60 kph, it can reach 17km/litre with AC. Without AC it does about 2km/litre better. When I got my 2004 Vitz, I was getting just 10 km/litre, but once I changed the spark plugs and the air filter, the mileage improved quite a bit. The Vitz (1999-2004) has a 40-litre fuel tank capacity. (Note: Some people mistake it for 45 litres and get wrong mileage figures). On the Vitz digital fuel gauge there are eight bars, so each bar corresponds to about 5 litres, which should help in fuel monitoring.
Spark plugs should be changed every 15,000km as per the workshop recommendation, but I would say 10,000 km is better. The Vitz 1SZ-FE engine uses 'NGK' BKR5EYA as factory fit, but 'DENSO' K16R-U are also compatible. Remember, spark plugs are one of the most important, but least remembered, components of the engine. Good plugs can help fuel economy as well as power output of the engine.