Amberjac Plug-In Prius: Hail the 130mpg car

Poking fun at the Toyota Prius has been a popular pastime. Owners including Cameron Diaz and Leonardo DiCaprio were getting nowhere near the claimed fuel consumption, and, most amusingly, the CO2 emissions are not even low enough to qualify for Gordon Brown's latest zero rate car-tax band.

So stop laughing and start gawping at the Plug-in Prius. Not that there is a huge amount to gawp at, as it looks just like the standard-issue hybrid.

And it has the same Prius ingredients - an efficient petrol engine, automatic gearbox, electric motor and additional battery pack. However, this Prius returns in excess of 100mpg in petrol-powered mode.

Simon Sheldon, managing director of Amberjac Projects of Grantham, Lincolnshire, which is behind the Plug-in Prius, points to a tray of batteries. "Those are the original nickel metal hydride batteries, which we have replaced with lithium iron phosphate."

These have seven times the capacity of the originals and the cars in which they are fitted get 27 times the energy. Plus, they are safe: Sheldon stabs a battery cell right through with a screwdriver without causing a fire. Indeed, the cell still works.

As well as clever batteries there is also a cleverer battery management system, which helps this modified Prius to travel 30 miles in electric mode (30 times higher than the standard car) and allows it to return up to 130mpg compared with 50mpg in petrol mode.

"We thought we could do a much better job than Toyota. Doubling the car's efficiency seemed achievable. I've worked in the automotive sector and in battery industry and there is a huge skills gap between the two. One does not realise what the other is doing. That's given us a huge opportunity.

"To get to this customer-ready stage has cost about £150,000," says Sheldon. That is the annual coffee bill for major manufacturer boffins.

At the rear bumper of the Plug-in is a flap which covers a socket. Unlike a standard Prius, you boost the batteries overnight on cheap-rate electricity (it costs only 21.9p to fill up), and that is the key to its remarkable urban performance.

Engage the gears and you are off at a whisper, powered by the electric motor. You can have climate control and soothing music without flattening the batteries. There is a pod in front of the driver that tells you what the situation is.

"Our monitor shows the battery condition and the throttle position," says Sheldon.

In addition, there is the Toyota's own colourful and mesmeric energy monitor, which tells you what the power units are doing. After a few minutes in a standard Prius, the engine cuts in to help the batteries, but the Plug-in just keeps on going, and will travel under pure battery power for 30 miles.

"We have had a problem describing what this car is," says Sheldon. "It isn't strictly a hybrid, or purely an electric. It is in between. We still have to warm up the engine, and for that we do need to burn some petrol so that the catalyser works efficiently."

We accelerate to more than 31mph and the petrol engine kicks in. The display, which was showing 999mpg, tumbles down to 52.5mpg. But that figure does not last for long, even though we are now overtaking on a dual carriageway. At 60mph we are doing 100mpg.

Sheldon points out that it is reading in US gallons so I have to add 20 per cent, meaning a remarkable 120mpg. And when you coast up to roundabouts, you are only on battery power.

So why do standard Prius owners achieve such poor consumption figures? "Their driving style needs to change," says Sheldon. "You need to anticipate conditions - there is no point in accelerating hard halfway up a hill. Toyota claims 65mpg but we have found that under normal driving conditions it is more like 48mpg."

Unofficially, Toyota has seen the Plug-in conversion and could not believe that someone had reverse-engineered its baby so successfully. It will catch up, but is years of corporate development behind.

Amberjac can now turn its attention to the hybrid off-roader Lexus RX400h, which gets Sheldon very excited: "At just over 30mpg it doesn't make sense, but we can easily double that figure and I believe it will look much more attractive."

He's right. We could glide through the streets without upsetting any of the anti-4x4 lobby or, indeed, any wildlife. As I ponder this, an irate goose ignores the car and mounts a prolonged attack on our photographer.

The Plug-in Prius is what the car should have been anyway, but at a price. Turn up at Amberjac with your Prius, and for £7,700 plus VAT you will have one of the most environmentally friendly cars in the world.

The hardware can all be removed and transferred to your next Prius. Amberjac is also introducing lower-powered battery packs for drivers who don't need the ability to go 30 miles without an engine, and these versions will cost several thousand pounds less.

If you can't afford this, Sheldon is hoping that local authorities and government will back up their green posturing with orders. Currently, the C02 output is reckoned to be about 60kg/km. When this is officially confirmed, it will unusually qualify the Plug-in Prius for zero rate car tax. And later in the year, Amberjac will switch the fuel to bio-ethanol.

The Plug-in is a deadly serious piece of kit, perfect for A-list celebrities keen to offset the CO2 of their Lear Jets.