There are lots of equations to deal with when buying a used car. Availability of Auto Trader divided by newsagents open at 7am on a Friday, multiplied by cups of tea in a cafe, for instance.

Or phone calls to shifty blokes who will only agree to meet you in a car park divided by how many examples of the car you're after are available on that day. That one never works in your favour.

But it's the technical equations that really get me going: equipment levels expressed as a fraction of the cost when new, engine size plus bhp plus fancy interior kit or, my favourite, list price divided by massive depreciation.

And there's little that depreciates as much as, or is as well equipped as, a top-end BMW, which is where the M535i comes in. Once this was the quickest car BMW made (140mph, 0-60mph in 7.4sec), and cost £17,950 new.

This model came before the M5 and the first M535i was the original M-car. It too was fast, agile and lethal in the wet, but most have either been stuffed into a ditch or artfully bent around a tree. The second-generation cars are more plentiful and easier to live with. And if you could work out a way of applying those daft equations to them, you would get impressive results.