Remember that Corvette ZR1 we told you about just before Christmas? The one that, with its 6.2-litre V8 putting out 620bhp and 595lb ft of torque, was set to become GM's most powerful car ever?

Well, it seems the General might have been a bit, erm, conservative about the power output of the ZR1. The car has just completed SAE certification and was found to produce - ready for this? - 638bhp and 604lb ft of torque.

Yowch. The science behind all the power, say the engineers at GM, is a new supercharger with a four-lobe rotor design that displaces 2.3 litres. That's a monster charger, that is.

Despite all the noise and fury, GM says you won't be spending all your time sheepishly pointing the wrong way down the road in a cloud of smoke.

"This car has road manners that will allow our customers to enjoy it on the streets as a daily driver, and on the track," says Chevy boss Ed Peper.

Though there's no word on economy yet, GM engineers claim 'the ZR1 will be the most fuel-efficient 600-plus-bhp car on the market'. Which is a bit like being the cuddliest machine gun on the market. We've got a feeling it won't sneak into the free congestion charge bracket, either.

And - almost as an afterthought - the GM guys have revealed the ZR1's top speed. 205mph. Welcome to the 200mph club, lads.