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Is it credible that the new Mercedes-Benz CKS 350 petrol returns 40mpg?

I was surprised by the fuel consumption claim of 40mpg combined for the new Mercedes-Benz CLS 350 petrol model. I think the old model 320CDI was 36mpg combined (37.2mpg on 245 rear tyres). I have a 5-year-old 350 petrol, which had a combined claim of 29mpg that is only achievable on long motorway runs. I am aware that much work has been done to improve the economy of petrol engines but is this credible?

Asked on 5 March 2011 by PJ, Wendover

Answered by Honest John
No. All it means is that they stuck it on a chassis dynamometer that replicated the EC driving cycle and managed to achieve the equivalent of 40mpg in the EC combined cycle. Whether a real person can really achieve that in real life is another matter entirely. That said, over 692.3 miles in a Jaguar XJL 3.0V6 diesel I recently averaged 41mpg at an average speed of 58mph, and that is better than the official combined figure of 39.2mpg. For a better idea of the true fuel economy of recent cars, consult and participate in the Real Life Fuel Economy Register at: www.honestjohn.co.uk/realmpg
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