By far the most significant part of this car lies under its bonnet. If you want a two-litre turbo diesel engine, you have a very wide choice, but the thing about this one - a development of the already familiar unit used in several models by Renault and its alliance partner Nissan - is that it produces 175bhp. Even for a two-litre petrol engine, that's not bad going, but for a diesel it's exceptional.It's not so much the maximum power figure itself that matters as the mid-range performance that goes with it. Maximum torque (a chunky 266lb/ft) is produced at 1750rpm, so strong acceleration is available to you whenever you need it. Even a mild tweak of the pedal is enough to take care of most overtaking moves, and if you really go for it you can gain unexpected speed very quickly.I say "unexpected" because the Laguna GT doesn't look like a performance car. The alloy wheels look good but don't particularly stand out, and nothing else about the body styling announces that this is anything other than a standard Laguna.It doesn't feel much different, either. The ride quality, for example, is very high - the GT wafts over most bumps, though it does have the characteristic French tendency to jar on the more sudden ones. The steering is precise without being sharp. Body roll is evident, but well-controlled.If you never made use of the power, you'd think that this was an above-averagely comfortable medium-sized hatchback. And you'd be right; if there's one thing the Laguna GT isn't, it's a sports car. Fine - it isn't meant to be. But if it's not set up that way, surely it's inadvisable to pump 175bhp through the front wheels?As it turns out, no. There must be more to the suspension than there first appears to be, because in fact the Renault chassis people have done a superb job of harnessing all that power. Without losing any of the ride quality, they have also managed to make the car accept what the engine can throw at it without losing composure.Oh, of course you can get into trouble if you try hard enough. Too much right foot work at the wrong moment can promote understeer, caught by the electronics as yellow lights start to flash on the instrument panel. But you can make very good progress on challenging roads without any of this happening. Only an especially clumsy driver could complain about the engine overcoming the abilities of the chassis.I can't say a word against the car's performance. It's as quick as anything needs to be on the public road (though whether you want - rather than need - more power than the Laguna provides is of course another matter entirely).There is, however, a penalty to be paid, and it's bigger than it might appear to be on paper. In the official combined fuel economy test, the GT dCi 175 achieved 47.1mpg, which is just 0.8mpg shy of what the 150bhp version manages. But there is no part of the economy test which takes performance motoring into account, and if you start making use of the GT's extra grunt the fuel consumption rises significantly.According to the trip computer, I managed just under 39mpg during this test. That's a long way from 47.1mpg . . . but on the other hand it's still good enough to let you drive for 500 miles between refills. And my figure takes into account all the times (there were several) when I took advantage of the GT's acceleration.In more gentle motoring, the official figure may well be within reach. Or, if you want to look at it another way, I can't think of many cars of similar size which would have covered the ground as effectively as the Laguna did while still squeezing 39 miles from a gallon of fuel.Standard equipment on the GT includes a satellite navigation system which is useful in its own right but creates a minor problem. The control unit on the centre console uses up the space which on lesser Lagunas acts as a cupholder; this means that the only holders left for front seat occupants are the two in the glovebox (not very convenient), though rear passengers have another, handier pair in the armrest.There are plenty of other electronic gizmos, such as the key card. The Laguna senses its presence and will lock its own doors - accompanied by a slightly irritating double hoot on the horn - when the card is taken far enough away. It will also unlock the doors as required when the card comes back in range.What it won't do, though, is let you switch on the engine simply because the card is on the premises. You have to insert it into a slot, and perhaps the most annoying aspect of the whole machine is that the first thing that happens when you remove the card from the slot is the appearance of a message on the instrument panel saying "Please Remove Card". Grrr.The handbrake is automatic too, in that it releases when you start pulling away from rest and engages when you switch off the engine. Alternatively, you can apply the brake using a dashboard switch, which is less convenient but at least gives you a feeling of control. I would have to be more confident of Renault electronics than I am, however, to be happy about putting my complete trust in a brake which relies on switches and signals rather than levers and cables. Engine 1995cc, 4 cylinders Power 175bhp Fuel/CO2 47.1mpg / 159g/km Acceleration 0-62mph: 8.4 seconds Top speed 140mph Price £22,550 Release date Details correct at publication date