Cost of road tax would be one of the smallest costs of running a W12, but perhaps your friend should check the simple stuff before kicking the tyres, sorry, that kind of things get me
He probably better off with something else
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2019 BMW M5 (my daily driver)
2002 Mark's D2 S8 FE
2020 BMW M340d xDrive (Anely's daily driver)
2000 FL D2 4.2 QS - SOLD 2011