Diesel may be slowly falling out of fashion but in pure economical terms it still does the business, with up to a claimed 56.5mpg combined from the 220d. That allows for a CO2 figure as low as 110g/km.
The three-cylinder though isn’t significantly less economical, though it’s worth remembering it combines its 47.1mpg figure with lower performance than the diesel. CO2 for the 218i is down to 123g/km at its lowest.
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As for the M235i, you can theoretically achieve 37.2mpg, with 153g/km of CO2. Our long-term M135i is achieving around 30mpg on average, so you can expect that as a decent day-to-day baseline with the potential for either more or less depending on how you drive.
Given the 218i and 220d are less likely to be driven hard than the M235i, it’s fair to imagine tyre costs may be more relevant to the latter. On 18-inch wheels the sportiest Gran Coupe isn’t excessively shod for this class, so £105 a corner fitted for Michelin Pilot Sport 4s (evo’s 2017 tyre test winner, from Blackcircles) doesn’t seem too bad, with Dunlop Sport Maxx RT2s (2018’s bronze award) at under £87 a corner.