If you want your family car to be faster than is strictly sensible, you've got a lot of options. These are two of the latest, and the best. The Mercedes-AMG GT 4-door S is among the fastest and most expensive. It has a 4.0-litre V8 making a whopping 630bhp, which is sufficient to push the GT to almost the 200mph mark (where conditions allow, obvs). And its fiercest rival is the Porsche Panamera, w
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