An engineer designs an improved automobile engine for a certain make of automobile. The previous design had an average highway petrol consumption of 10.0 l/km (litres per 100 kilometres) with a standard deviation of 0.5 l/km. The new engine has an average highway petrol consumption of 9.8 l/km, based on a sample of 3600 cars of the model with the new engine. A test of significance for the mean petrol consumption of the new engine is conducted. Based on this sample, the Z-statistic associated with this test of significance is (no units required). Round to no decimal places.