For the past 14 months, I have driven my car topless most of the time. As a matter of fact, I only put the top up when it rains and when the temperature is inferior to -5ºC/23ºF. So far, I haven't seen any significant increase in petrol-consumption when the hood is down. I suppose huge square American cars of the 50's and 60's with roomy back-seats consumed more with the top down due to a greater change in aerodynamism but a small carefully designed car such as the SLK with ultra-aerodynamic lines cannot be that much more petrol-greedy when driven with the top down. But I am no expert. Just a guy who's madly in love with his car!
