AAddict is about right, but it is possible to trigger the ACT beyond 80mph I find, I've certainly seen it at 85, maybe a touch higher. The problem is simply that the torque required to maintain a steady speed at some point exceeds the limit at which it switches back to 4-cylinder mode. You can "coast-and-burn", i.e accelerate to 85 or so, then come off the gas somewhat and allow the speed to drop down to seventy, then repeat, but I find this a very tedious way to drive.
What Mart!n suggests tends to work better, I tend to keep the DSG in manual most of the time anyway, and you find that in traffic on the motorway that's moving between 50 and 70mph, you're better off in 6th rather than 7th. This means ACT activates more readily. However, you've got to keep an eye on instant consumption, because you may actually use less fuel in 7th with 4-cylinders than in 6th with 2! There are crossover points that will inevitably take a while to learn.
I'm finding that on my regular commute (30 miles of the M1, 10 miles of the A42, 2 miles of ordinary roads) my average consumption is increasing day by day. I'm now finding it easy enough to get 45+mpg if the traffic is typical rush hour type stuff, running between 50 and 70. However, when the road clears and you get the opportunity to "make some time" it's extremely easy to knock the consumption right down to 30mpg.
I was running an Audi A3 prior to this with the 140bhp diesel engine, and that was considerably more economical if you like to cruise on a motorway at 85-95mph, it was only if you consistently went above that that it would dip down to 35mpg or so. I once did a memorable 100mile trip where I got a 50mpg average consumption, and a 70mph average speed. You'll never see that in a BlueGT.
Don't get me wrong, the BlueGT is still a cracking car, and I prefer it to the Audi a vast amount, I just want to make sure that anyone reading this thread doesn't expect to get comparable economy to an equivalent 140bhp diesel, even if that's what the published figures imply. It's also worth remembering that if you're in town a lot more than me, you may well do better than the equivalent diesel, as it will happily potter around at 20-30mph in 2-cylinder mode most of the time. Added to this, stop-start is a much more worthwhile technology in petrol cars - as I understand it, the fuel required to restart a petrol engine is less than that required to restart a diesel. Lastly, you have to factor in the difference in price between petrol and diesel.
The other thing I disagree with the review about is the sound of the engine when it's in 2-cylinder mode. You can definitely hear the difference, especially at lower speeds, it sounds great, quite thrummy, a bit like an old 2-cylinder motorbike, or even those 650cc single cylinder KTM dirt bikes (I might be slightly optimistic here)!
