To put this in perspective, the best you could expect from a BMW 325 TDS Touring - the Audi's nearest rival - would be 38.2mpg*. And tops from a Mercedes-Benz C-class 250 Estate is 36.7mpg*. To find cars capable of matching the Audi's fuel saving prowess you have to look to less prestigious metal. And even here, you'll struggle. A Fiesta diesel, for instance, posts 46.3mpg combined.
It's no surprise then that the Audi's closest economy rivals are its Volkswagen Group stablemates that share the same ground-breaking direct-injection diesel engine. Ground-breaking because Volkswagen was the first manufacturer to harness the fuel saving potential of DI while achieving acceptable levels of performance and refinement.
With the 110bhp unit fitted to our A4 - which is distinguished from the lesser 90bhp model by the red 'i' on the TDI badge - that achievement has been taken to new heights. The bald figures tell much of the story: 0-62mph in 11.5 seconds, a top speed of 119mph and 30-50mph acceleration to eclipse even the 2.4-litre petrol V6. At the same time, you get close to 50mpg at the pumps with - in my case - no effort.
What the figures don't reveal, though, is the Audi's refinement on the move. At speed it's hard to differentiate it from a petrol car - testimony to the engine bay and bulkhead being swathed in sound-proofing. Mpg statistics alone also fail to convey the magnitude of the fuel savings on offer. In three months, the A4 TDI has cut my fuel bill by 40% - admittedly against a petrol V6 predecessor, which averaged 30mpg.
It means that my typical 3,000 miles a month are now costing ú182.89 at the pumps compared with ú303 before. And that points to an annual saving of almost ú1,500 - too much for a new department whose budgets are under constant scrutiny to ignore!
On the road, the A4 is proving a serene long distance companion - if rather uninvolving to drive. It lacks the communicativeness of, say, a BMW 3-series. The steering is a little over light and lacks feel, and both the clutch and brakes have a fierce bite which makes smooth progress a question of familiarity. Nevertheless, such grumbles are far outweighed by traditional Audi virtues.
The A4's safe-like build quality, sculptural styling, and beautifully crafted cabin lend it an ambience few rivals can match. All this - and the economy of a supermini. So far, so frugal.