IT would have sounded like a tall story even a few years ago - a luxury saloon with a top speed of nearly 150mph that can notch up around 50 miles to a single gallon of fuel.

But now we know it's a fact, albeit a rare achievement. The new Audi A7 3.0 TDI Ultra with its sleek fastback shape and opening tailgate has lower CO2 emissions than my wife's humble five-year old Fiesta and is more economical than the average 1.6-litre runabout.

Ultra is the tag Audi attaches to its super-clean versions with diesel engines tuned for economy mated to an efficient seven-speed S tronic automatic gearbox and stop-start ignition.

From the driver's point of view it feels and looks like any other A7 Sportback - smart, swift and relaxed. The main difference is fewer trips to fuel up.

The 3.0-litre V6 turbo diesel pushes out 215bhp providing a 149mph top speed and acceleration to 62mph in a fairly swift 7.3 seconds.

This version isn't endowed with quattro four-wheel-drive but manages perfectly well with front drive - it's cheaper to manufacturer, and lighter so more fuel is saved.

Emissions are amazingly low at 122g/km and the official combined claim is 60.1mpg, although the 50mpg mark is closer reality.

There are acres of space in the smart A7 cabin with wide seats, stretch-out legroom front and rear and an enormous hatchback boot with rear seats that can be folded down.

The luggage platform is quite shallow but it extends well within the car allowing 535 litres of cargo room.

Access to the rear seats for passengers is slightly restricted by the lower roofline dictated by the coupe-like profile.

Facia design is familiar high quality Audi with clear dials and high-grade plastics. hNevertheless, the heating and air con controls are needlessly complicated, but Audi owners will be used to that.

Ride and handling is controlled and comfortable with a slight sporty edge that means little body-roll and high levels or adhesion.

Road surfaces have to be pretty poor to affect the A7's composure and you notice the lack of four wheel drive only when accelerating hard out of bend when the car's substantial torque is fed through those front wheels.

The twin clutch automatic gearbox is as good as it gets with fast, almost undetectable changes that make light of long journeys or heavy traffic.

Steering has more feel and weight than Audis of old, but it's still not the most communicative of systems when compared with either BMW or Jaguar.

The A7 has recently undergone a few small cosmetic changes to keep the four-year-old design fresh.

More aggressive squared off bumpers and fared in exhaust are among the most noticeable modifications, along with a slightly re-modelled front grille.

The price is £46,855, but MMI sat nav and head-up display - a really useful tool - is a £2,345 extra.