I read with interest your article on the Mercedes SLK200 BlueEfficiency AT, mostly because I have just purchased a C200 BlueEfficiency AT which has the same engine. I have owned several diesel cars and was encouraged to switch to a petrol engine by the excellent fuel economy figures published both by Mercedes-Benz on its website (a combined-cycle figure of 6,4 to 6,9 litres/100 km), and by your magazine, with a stated 7,6 litres/100 km. In practice, however, I’m averaging 11,0 litres/ 100 km in a mixture of city (75 per cent) and extra-urban (25 per cent) driving. The Mercedes dealership from which I purchased the car said that the engine was correctly tuned and suggested that I could expect an improvement after 10 000 km. My question is – how do you calculate fuel consumption in your tests? Your CAR Guide notes say that you use a combination of your own road-test figures and the manufacturer’s figures, but Mercedes’s figures are lower by about 60 per cent than those I experience, and yours by nearly 50 per cent. I’m awaiting a response from Mercedes-Benz to a query I submitted via its website, but this is too large a discrepancy to allow for the slightly optimistic figures that motor manufacturers publish when they do tests in ideal conditions.
PRAMOD DAYA – Kenilworth
Manufacturers normally publish fuel-consumption figures as achieved on the new European drive cycle (NEDC). This cycle is run on a chassis dyno under controlled conditions and is mostly used for emissions-related purposes (EU legislation). Unfortunately, this cycle is very conservative and involves gradual acceleration and a low average speed. Also, no auxiliary equipment such as the air-con is switched on. Therefore, these figures are difficult to achieve in real-world driving conditions.
CAR magazine modifies this figure to be more representative of real-world driving conditions due to experience gained over the years of testing cars. Included in the latest road tests is a fuel-route figure where the test car is driven in a city and extra-urban environment to give the reader more insight. The problem is that fuel consumption can be compared between vehicles only under controlled conditions such as the NEDC cycle. Any other means will give the reader an idea without being scientific. It is very difficult to tell if your car is using more fuel than it should (the dealer found nothing wrong) as your exact drive cycle, auxiliary status and driving manner (acceleration/braking) are unknown.
In short, city driving (stop/start) is a killer for fuel consumption. The technology of turbo charging and direct injection has brought petrol engines closer to diesel in the consumption stakes but diesels are still more fuel efficient for various reasons. We would suggest you take the car on a long run at steady speed (100 km/h) to see if the fuel-consumption figure improves significantly compared with your daily drive cycle.
