The Audi A4 3.0 TDI and the Volvo S90 fail to meet their official MPG figures in a real-world economy test, but how do others fare?

Ah, downsizing. It'll save us all, right? Take a big, naturally aspirated engine, make it smaller and stick a turbo on it for greater efficiency. Et voilà, hey presto, etc, a vast improvement in fuel economy and emissions. Right? Right?

Ah. Well. M’colleagues over at What Car? have a thing called True MPG. I don’t quite understand what happens, but I know it takes place at Millbrook Proving Ground, where they employ seriously serious engineers who possibly but by no means definitely wear white coats while strapping heinously complicated and even more expensive testing equipment to the back of a car to find out what kind of economy and emissions you should actually expect from a car should you buy it. The alternative, of course, is that you just trust the NEDC economy and emissions tests that are run by manufacturers, in labs, whose results are notoriously hard to replicate in everyday driving.

Anyway, the most recent set of True MPG results is out, and among them are some recently downsized cars. Take the Volvo S90, from a manufacturer whose vice-president of R&D said big diesels were “gas guzzlers which we are never going to see in our cars again”. His 2.0-litre four-cylinder S90 D4 diesel just returned 39.9mpg on the True MPG cycle – a shortfall of 37.8% on the official combined figure and 2.1mpg worse than that of the six-cylinder Audi A4 3.0 TDI.