Saving weight is like gaining free horsepower. Your car’s performance qualities are governed not just by its power, but by its power-to-weight ratio – so if, for example, you throw away your rear seats and spare wheel and a few other weighty things, your car will feel a bit quicker. If you follow this thought process through to its logical conclusion and make weight savings everywhere you can think of, the transformation can be dramatic; take this Fiesta RS Turbo, for example, which boasts a featherweight all-in figure of just 875kg. This has been achieved by gutting the internals of the doors, bumpers and tailgate, throwing away all the soundproofing and carpets, swapping out the bonnet for a fibreglass item, and replacing all of that heavy window glass with slender…