A DIESEL Audi with Prius-rivalling fuel economy has been introduced in Australia. The A3 Sportback TDIe is not a revolutionary concept, but uses a series of small modifications that help it save fuel.

Its official fuel economy figure of 4.5L/100km is just 0.1 litres higher than the petrol-electric Prius hybrid and while the Toyota is leaner in stop-start driving, the Audi uses less fuel in higher-speed driving conditions.

Audi demonstrated this recently in the World Solar Challenge late last year, recording an average fuel economy figure of 3.3L/100km in the cross-country drive, to be beaten only by a diesel Hyundai i30.

The new A3 TDIe costs $38,900, which means it is the second-cheapest model in the A3 Sportback range, sitting above the $37,200 entry-level car.

It runs the VW Group's 1.9-litre turbo-diesel, which is not currently used in any other Audi model in Australia, but it is available in Australia in the Volkswagen Golf and Skoda's Octavia and Roomster.

It's a direct-injection four-cylinder that uses a variable-geometry turbo to produce 77kW at 4000rpm and 250Nm at 1900rpm.

This compares to the more expensive 2.0-litre turbo-diesel engine available in other Audi/VW Group models that produces 125kW and 350Nm.

The A3 TDIe is no performance car and takes all of 11.7 seconds to accelerate to 100km/h. That seems like an eternity, but is actually faster than the positively pedestrian 1.6-litre petrol A3, which takes 13.2 seconds.

The A3 e is only available with a five-speed manual gearbox. Audi engineers have made some small changes to the transmission in order save some fuel. It runs higher third and fifth gears in order to put less load on the engine at higher speeds.

Audi has also introduced a system that helps the driver save fuel by indicating which gear they should be in. This information is shown in the information display in the middle of the instrument cluster.

The A3 e also features a modified engine control unit that has been tuned for economy over responsiveness.

Audi has also concentrated on non-powertrain changes in order to reduce the A3 TDIe's fuel consumption. The company has selected Michelin tyres with ultra-low rolling resistance, thanks to their tread design and the use of Silica.

The wheels have also come in for attention. Audi has fitted the A3 e with steel wheels and plastic wheel covers that run an aerodynamically efficient design with no spokes as such.

The special model also runs lower-riding suspension in order to improve the aerodynamics.

It all contributes to an official fuel economy average, using the ADR81/01 formula of 4.5L/100km, which equals 119g/km of C02 emissions. At that rate, the A3 TDIe is able to cover 1200km on a single tank.

For the record, the e version uses eight per cent less fuel than the regular 1.9 TDI A3, which is sold in markets other than Australia.

Break down the figures and you can see where the A3 e shines.

It uses a respectable 5.8L/100km in the city driving sector of the fuel economy test, which is good, but sips just 3.8L/100km on the country driving cycle.

That is quite amazing given that apart from the fuel saving measures mentioned, the A3 TDIe is just like other A3 models.

It is 4292mm long, 1765mm wide, 1423mm tall and weighs 1320kg. There is 370 litres of boot space, which is the same as all other A3 models as there is no battery pack to reduce the practicality - as is the case in a hybrid.

The A3 TDIe also retains the 60/40-split folding rear seat design which expands cargo capacity to an impressive 1100 litres with the rear seats down.

It comes standard with a full suite of safety gear including front side and side curtain airbags, electronic traction and stability control and anti-skid brakes with brake assist.

Inside, there is dual-zone climate-control, a single-CD sound system, head retraints for all five seats and a leather-wrapped steering wheel.

Surprisingly for a model from a prestige brand like Audi, cruise control is not standard and costs $750. Picking one with metallic paint adds a hefty $1300.

Other options include $800 rear parking sensors, a $300 front armrest, leather trim for $2900 and a sunroof at $2950.

Despite its impressive efficiency, Audi is not expecting big things from the A3 TDIe, which was introduced to the public at the Australian International Motor Show in Sydney last October.

Going on orders placed since the show, Audi is calculating it will sell just five a month.

Audi Australia says that in future it will introduce some of the A3 TDIe’s fuel saving features into other models in its range.