With only two engines currently available you won’t need to burn many calories on deciding which model will best suit your idea of low running costs.
On fuel consumption, CO2 and related taxation alone, the diesel looks to be the better pick: both standard-wheelbase and LWB 50 TDIs are capable of 50.4mpg on the combined fuel economy cycle, with CO2 of 154g/km. That will result in a £515 first-year VED bill and £450 a year thereafter, owing to a list price of over £40,000. BIK for 2018-2019 is 34 per cent.
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In contrast, the petrol 55 TFSI (again irrespective of wheelbase) achieves 36.7mpg combined and 175g/km of CO2. Its own tax figures are £830 in first-year VED and £450 thereafter, and 35 per cent in BIK for 2018-2019. In other words, the petrol will cost you more both to fill and to tax.
There are no solid indications on the horizon as yet how taxation and systems like the London congestion charge may change relative to petrol and diesel models, so for the time being the diesel - which also costs less to buy than the petrol - looks like the more affordable car to run.