Well, this is embarrassing. For years we in the motoring press have been force-feeding you the good news about diesel-powered luxury cars: that they're surprisingly refined, extremely fuel-efficient and a whole lot more fun to drive than their petrol equivalents.
European Motor Distributors, importer of Audi into New Zealand, has made the argument easier in the last decade by achieving price parity between diesel and petrol models.
Note the past tense. With European emissions regulations set to really challenge diesel technology in the medium term, many makers are taking another look at what can be achieved with petrol powerplants.
The Volkswagen/Audi Group is a prime example. By combining sophisticated direct-injection systems with turbo and supercharging (sometimes both together), the German giant is extracting an incredible combination of performance and economy out of petrol engines.
Which brings us to the new Audi A6. Previous generations have been cool-looking, high on quality but a bit stodgy to drive, which has made diesel the obvious choice.
The latest model - still swish and beautifully made, but lighter and more driver-focused - makes things more complicated.
As always, the TFSI (petrol) and TDI (diesel) versions cost the same: $143,900. With 220kW/440Nm and combined economy of 8.2 litres per 100km, the TFSI is more powerful than the diesel and almost as torquey - although its peak pulling power is available at 2900rpm, compared with the 500Nm at 1400rpm of the TDI. It drinks 2.2 litres more fuel per 100km, but the extra greenhouse gas generated by burning diesel closes the CO2 gap a little - 190g/km for the TFSI versus 158g/km for the TDI.
The A6 petrol rockets to 100km/h in 5.5 seconds, compared with 6.2s for the diesel.
The TFSI leaps off the line and rushes to the redline in each gear, whereas the TDI lags initially, then surges forward as the diesel engine hits its stride. The TFSI is exciting, the TDI impressive.
The new A6 is on average 80kg lighter than the old and despite Quattro all-wheel-drive being standard, Audi claims it's still lighter than rear-drive rivals from BMW and Mercedes-Benz. That pays dividends in fuel efficiency but also handling. There's a nimble feel to this A6. And it's the TFSI, with less weight over the front wheels, that best exploits the new car's chassis smarts. The crisp petrol engine also seems to get the best out of the seven-speed gearbox.
The Audi A6 is still no rival for a BMW 5-series or Mercedes-Benz E-class in terms of driver appeal. But for those who like the undeniable bling of that Audi badge and all-wheel drive, the new car certainly claims its own company carpark.
Love the effortless torque of the 3-litre diesel and there's no question it's the more cost-effective on fuel - but the great thing about the new A6 is that it does justice to the extra throttle response and high rev-ability of that fantastic 3.0-litre supercharged petrol. That's the one I'd go for.
The bottom line:
Effortless diesel or sprightly petrol? Price parity and an impressively broad range of abilities in new A6 mean that the choice has become much more difficult.