The fuel-thriftiness of small diesel engines has been matched somewhat by improved petrol technology, to the point where strong oil-burning advocates like Volkswagen dropped diesels in its small Polo range in favour of direct-injection petrol units.
The punitive Road User Charge in New Zealand doesn't help the case for small diesels when comparisons are made with petrol units in the same-sized car.
The owner of a diesel variant of the Kia Rio, for example, has to front up with around $44 for every 1000km covered. That's because, unlike petrol, diesel isn't taxed at the pump.
Clock up 80,000km in the 1.4-litre CRDi Rio diesel and you pay about $3500 extra for doing so. Do the same distance in the 1.4-litre petrol version and you don't.
There's the argument. A real-world test of city and urban driving between the two would almost certainly result in a win for the diesel - before the RUC was factored in.
It happened a couple of years ago in the Energywise Rally, a 1763km Auckland-Wellington round trip. The Mini diesel won the fuel-thrifty crown hands down, recording 4.2-litres/100km (67mpg) and using 74.5 litres of fuel costing $92.34.
But the petrol-electric Honda Insight S won overall. It did 4.6 litres/100km (61mpg) and used 81.7 litres of fuel costing $147.83.
The hybrid Honda's CO2 exhaust emissions were rated at 107.8gr/km against the Mini's 110.1gr/km. (The petrol-electric Toyota Prius was the CO2 winner with 102.1gr/km).
But the RUC of $77.57 pushed up the Mini's overall cost to $169.92. Look at it another way: RUC of $77.57 is around 83 per cent of $92.34, the cost of the Mini's fuel. So 83 per cent of the cost of the journey was a tax.
Carmakers worldwide are downsizing engines, making them cleaner and more economic. The above equation makes a case for at least downsizing the RUC on small diesel engines.
Kia claims the oil-burning Rio hatchback ($24,990) is good for 4.3 litres/100km, with a CO2 rating of 113gr/km. Helping fuel economy is the carmaker's EcoDynamics package, things like stop-start technology, gearshift indicator, and low rolling resistance tyres. Kia also claims the design of the Rio's front grille cuts drag. The 1.4-litre engine delivers 66kW at 4000rpm and 220Nm of torque between 1750-2750rpm and is mated to a six-speed manual gearbox.
It's a sweet drivetrain but, like almost all small turbo-diesels, needs stirring along at low speeds. Coast through a give way sign in second gear at low revs and you will almost always need to chop back to first to get up and go with any urgency.
But there are not many trade-offs with the Rio. Its cabin is spacious, overall refinement is very good, so is noise deadening.
The handling is predictable, but steering is not as sharp as some of its rivals. Ownership will be largely pain-free with Kia's five-year warranty.
Like all Rio models the CRDi features stability control and Hill Assist Control to prevent it momentarily rolling backwards when pulling away up steep gradients.
Standard features on the Rio CRDi include daytime running lights for increased safety, reverse warning sensors, heated/powered door mirrors, remote locking with in-built car alarm, a height-adjustable driver's seat, two-way steering column adjustment, split-folding rear seats, trip computer, Bluetooth and a MP3-compatible/radio/CD audio system with USB and AUX ports.By Alastair Sloane Email Alastair