This is Volkswagen’s moon-shot car. We’ve just spent the day poring over it and have come back amazed. Welcome to the madly boffiny XL1.

It looks like a mini-supercar, it’s as specialist as a Veyron, and it’s certified in the official tests at 314mpg and 21g/km CO2. Which means it practically runs on thin air (it actually tested at 340mpg, but the officials imposed a rounding error - from 0.83/100km to 0.9). Yet it’s comfy and decently roomy for two, has air-con and navigation, and will cruise at 100mph.

It’s got a one-piece monocell RTM carbonfibre tub, with carbonfibre panels. That’s just like a McLaren P1. The drivetrain of the XL1 has P1 principles too: a mid-mounted engine, rear-drive, with a seven-speed DSG gearbox, a boosting electric motor and a plug-in hybrid battery pack. Except instead of a turbo V8 petrol engine, the XL1 has a 0.8-litre two-cylinder turbodiesel.

See more pics of the Volkswagen XL1

To be fair, that 314mpg figure includes a a few miles running on battery power obtained from the grid, depleting the battery. But if you didn’t plug it in and ran it as a simple hybrid, the engineers say it’ll still go through the test at 141mpg. The diesel tank is just 10 litres, but about £1 of electricity and £3 of fuel should get the car 300 miles.

The diesel engine develops 48bhp, the e-motor 27bhp. They can run independently or together. Performance is only equivalent to a basic supermini, but the XL1 should be fun to drive, with its mid-engine balance, very low centre of gravity, racecar-type front suspension and sheer agility - it’s so light it doesn’t even need power steering.

VW have said it will build a production run limited to just 50, but if there’s demand for more they will do them. But they won’t name the price. Having seen what it’s made of and how it’s made I’d say they ought to charge supercar money. But they can’t charge supercar prices for a car that takes 12.7 seconds to get to 62mph. It’s going to be a huge money-loser.

So why the heck have they done it? As a technological beacon for the Group, and as a testbed for new technologies. Not just for the carbonfibre tub or extreme aero either. In fact the exact same driveline will be put in the nose of a Up! next year. And with double the engine (a 1.6 TDI) the same thing will come in a plug-in hybrid Golf. And on the day we saw the XL1, Audi announced the A3 e-tron, which has the same hybrid system but a 1.4 petrol engine. Petrol means the A3 e-tron can be sold as an alternative to the plug-in Prius for people who actually like cars.

The whole plug-in architecture is scaleable and modular, and the VW group is convinced it’s the most viable alternative drivetrain for the future.

There’s carbonfibre all over this car, and everything else is specialist and lightweight. Lovely little wishbones in the suspension, lightweight dampers, carbon brakes, magnesium wheels, carbonfibre one-piece seats, a dash of compressed chopped wood fibres, the list goes on.

Net result, a weight of just 795kg, which is the same as their mental target, the Mk1 Golf. Now that Golf was a four-seater, but it was slower and far less safe and economical than the XL1 and much more sparsely equipped.

If you want a Top Gear guess, the price will be £50,000-plus, and even that would be far too low for VW to make a profit on it. Anyway, hang on for the Geneva show and they’ll tell us exactly. We’ll have a drive of it by then too.

Meantime, do you like the idea of a rocket-science economy car, or is it wasted effort?