SOMETIMES a car manufacturer reveals a new version of one of its cars and you are left scratching your head, wondering just where the changes have been made.

At the other end of the scale are cars like the new Toyota Prius, in which every part is either lighter, smaller, both or just better, with an all-new chassis allowing more interior space but a lower roof height and better aerodynamics. The overall result is a whopping great jump in efficiency. It's as if it has had a brain transplant.

The fourth Prius took five years to design and engineer, and by the time it arrived in the showrooms it was facing a sales threat from another hybrid in the Toyota stable - the cheaper if smaller Auris.

But the Prius remains the flagship hybrid for Toyota, the technological leader and the one where all this hybrid wizardry all started.

It has now got a whole lot more clever. For instance, the battery has been reduced in size and relocated beneath the rear seats to provide for a 502-litre boot on cars with a tyre repair kit, a new automatic parking function can parallel-park it and an air-conditioning function detects how many people are in the car and where they're sitting, closing off those vents that aren't needed.

The first thing you notice when driving is how quickly the electric motor kicks in to save you fuel. The days when the hybrid system informed you far too frequently that the electric-only mode was unavailable now seem to be a thing of the past.

That's just the opening salvo of a series of improvements. Faster steering, a stiffer body, suppler suspension and a more motor-focused hybrid drivetrain make the new Prius a creamy cruiser on electric-only power drive.

As with all hybrids of this type, the driving experience soon becomes an eco game in which you watch how the energy and fuel is being delivered - electric, petrol or a combination of the two - and how you might vary your driving style to gain the optimum miles per gallon.

You are encouraged, as the player in this game, to default to the electric-motor-only mode as often as possible, and where acceleration is need then lift off the pedal as soon as possible to re-engage the electric motor.

Your success is marked by a comprehensive range of bar graphs, eco scoring and mpg figures.

Gentle braking and smooth acceleration, and maintaining a constant speed, all help to eke out extra drops of petrol.

My eco score peaked at 97 per cent, while the best fuel consumption figure I achieved was 77mpg - still a little way off the official 85.6mpg figure but mightily impressive nevertheless.

Improvements to the petrol engine and the aerodynamics make the Prius about 14 per cent more economical than before on the motorway. The engine will also now cut out at up to 68mph, helping save fuel on downward runs, when the electric motor will take sole charge again.

It comes as no surprise that taxi drivers in particular have embraced the Prius.

Hybrids often achieve better mpg figures in urban surroundings, and tax-focused purchasers will just love the benefits. Retail buyers might also view it with fresh eyes now that the fuel economy is is so much better and there is no requirement for plug-in charging.

The new Prius retains the established 1.8-litre VVT-i petrol engine but it benefits from a drive assist system to provide a more responsive driving experience.

The car’s regenerative braking system has also been improved, with a new active hydraulic booster which gives quiet performance and controllability together with a natural braking feel.

Since the launch of the original Toyota Prius in Japan in 1997, and its subsequent introduction in the UK two years later, it has maintained its status as the hybrid standard bearer. The fourth generation Prius is achieving even greater advances.