Audi makes some excellent vehicles. I haven't driven most of them but the A3 is a favourite of mine, although the current outgoing one is dated.

I don't understand the whole carbon build up thing. Drive the cars like they're meant to be driven, all should be well. There's a line between abuse and use, and I think "use" is good for a car. Given my rural location, my X3 sees typical European hwy speeds (120-160km/h @ 2750-4750rpm) for extended periods every week and usually when I need to overtake a vehicle on a backroad, it's redline in a couple gears with some frequency.

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2014 Mazda 3 GT: Ø Consumption: (click for details)