The barometer here is twofold: how do the latest Cayman’s running costs stack up against the previous one’s, and how does it compare with its rivals.
That’s a gauge for any car, of course, but doubly pertinent for a sports coupé that has been separated from two of its cylinders.
Buy the standard Cayman and fit it with the PDK gearbox and Porsche claims you will see 40.9mpg combined and 158g/km CO2. That’s 5mpg and 25g/km more efficient than the car it replaces and a similar amount clear of BMW’s M2.
However, buy the car with a manual gearbox and the larger engine and the advantage narrows. At 34.9mpg, the S’s official average is only 3.5mpg ahead of the old 3.4-litre GTS’s.
At MIRA last year, the even more powerful GT4 returned 9.4mpg during track use; 10.3mpg from the new S shows just how immoderately thirsty a turbo engine can still be.
During TrueMPG testing, the 718 averaged 28.4mpg, which isn’t awful but neither is it a cast-iron economic reason for preferring the car to a nearly new example of its larger-capacity predecessor. Our test car’s 54g/km reduction in CO2 emissions over the GT4 encapsulates the incentive to Porsche as an organisation. But the £270-a-year reduction in road duty it represents hardly seems persuasive recompense for a repeat buyer.