The needle on the Calais' fuel gauge is wrapping itself around the 'empty' stop.

It's been like that for 45 minutes. If this were Star Trek, Scotty would be calling Captain Kirk and explaining how she'll not take it any more. My fingernails are gnawed back to the first knuckle, and counting. There's no doubt: 6.0 litres of Holden V8 will be sucking vapour any time soon.

Forty kays back I stopped at a servo and took out some insurance - a 4.5-litre plastic jerry can, with unleaded to suit. It's an extra 50 kays in a container, basically; a preventative measure against some undignified Keystone Cops story ending, halfway along Melbourne's Citylink motorway.

I'm about to fling the Tom Tom out the window, too (ten hours on the road can make anyone a mite intolerant). The Calais needs a drink now, the way a junkie needs the next hit. And I need not to run out of gas. Twice in the past 10 minutes I've trusted the British uppercrust voice of Kate Winslett to deliver me to the closest servo here in inner Melbourne. But our relationship is about to DNF. The first Winslett-purported servo was actually a construction site; the second, a car wash cafe (closed, of course).

So the window's coming down. My 'irreconcilable differences' with Kate are about to culminate in gravel rash. She's still gibbering directions, oblivious to her imminent demise. So long, baby, I'm thinking, it's been emotional. Then a ratty old 7-Eleven looms, and all is forgiven. The Calais burbles onto the forecourt, bug-spattered but a winner, as if it could go another ten rounds with the macadam, unassisted. Since the trip computer's indicating 899km, I almost take it around the block, just to prove a point. Then sanity prevails.

The Calais slurps up 79.6 litres of unleaded (says the bowser) until it's literally overflowing, the way it started the trip. Not bad for a car with an alleged 73-litre tank capacity (the trip computer reckons it's drunk more like 80.47 litres). Here's the kicker, and it's not likely to impress too many regional service station owners: The last time I filled it up was in Sydney's northern suburbs. That's Sydney to Melbourne on one (admittedly very, very brimmed) tank, returning an incredible 8.9 litres/100km. Yes, from a 6.0-litre V8. That anorexic fuel figure is in the Camry ballpark, from a car with 150 more kilowatts and more than 300 additional Newton-metres on tap when you need them.

The trip cost a mere $94.64 in fuel - well under the door-to-door cost of flying, even by the cheapest Qantas fare, especially after factoring in the additional costs, including parking at the airport, taxis or rental cars at each end.

How is this possible?

This is the first independent, serious evaluation of GM Holden's new Active Fuel Management (AFM) system. AFM is the first real environment/economy initiative from Holden since the Car-ocalypse nearly sent the big three American car makers to the wall (it still might). AFM passed, obviously. This car is a guilt-free V8 - maybe the first truly guilt-free Aussie V8 - at least on the highway. You still get the performance when you nail it (minus 5kW), but you lose the formerly intrinsic high fuel consumption when you don't.

AFM - also known as DoD (Displacement on Demand) or Cylinder Deactivation - works by turning your regulation 6.0-litre V8 into a 3.0-litre V4 when a pre-defined set of low-load and other conditions are met. It does that by shutting down every second cylinder in the firing order (that's cylinders one, four, six and seven for pub trivia competitors).

It's been reported elsewhere that AFM works solely by shutting down the fuel to those cylinders. Not so. In fact, the system shuts both the cylinders' inlet and exhaust valves when it activates. So nothing gets in; nothing gets out. Those four cylinders are, literally, taken out of the whole thermodynamic loop.

Doing so reduces what engineers call the 'pumping losses' experienced by the engine. Engines aren't pumps. A pump consumes the energy from a motor and does work on a fluid or gas - for example by filling the tank on your workshop compressor with compressed air, or jamming hydraulic fluid under pressure into your power-steering rack. Car engines, by comparison, extract the energy in the fluid (fuel/air mixture) and use it to drive the crankshaft around. Very different. Any energy used to suck mixture into the engine, or pump exhaust gasses out, constitutes a pumping loss - it's energy that could otherwise be used to drive the crank, but is actually wasted moving the fuel/air and exhaust through the system.

David Eliot, Holden's managing engineer for V8 calibration, explains: "AFM reduces pumping losses. That's really where the efficiency comes from. When you're just cruising along normally in a V8 the [inlet] manifold vacuum might be about 60kPa [8.70psi] below atmospheric pressure. When AFM activates, that vacuum drops to 30-40kPa [4.35-5.80psi], so the engine has to do a lot less work pulling the manifold pressure down."

Every second cylinder is shut down so that the firing pulses - the little rapid-fire bursts of effort that actually drive the crank around - remain evenly spaced, and the engine doesn't vibrate unevenly and otherwise run like a dog.

AFM doesn't activate at idle (idling on four cylinders is too rough) and it doesn't activate above 3000rpm. Above 3000 it's technically too difficult to switch it on and off, given the speed things are happening inside the engine (3000rpm is 50 revs per second). It's also redundant, seeing as above 3000rpm the driver wants mumbo, not economy, and there's only so much power and torque a 3.0-litre engine can deliver.

AFM also doesn't engage in first or second gears.

In third to sixth there are variable rpm/load 'windows' for AFM to do its thing. The biggest window is in sixth; the smallest in third.

The system works on oil pressure. The hydraulic lifters on cylinders one, four, six and seven collapse when oil pressure is fed to them by solenoid-controlled valves. The rollers on the bottom end of the lifters still follow the cam profiles, but the tops of the lifters don't push the valves open - in this condition AFM is active, and every second cylinder is out of play.

It's a good arrangement because the default state for the lifters puts the engine in V8 mode. That means start-ups take place on eight cylinders, which is smoother (there's no engine oil pressure on start-up). It also means that if the AFM system becomes defective, the engine turns into an ordinary V8 and you won't be left to limp home on four cylinders.

The test vehicle we drove was a pre-production car, bereft of the badging and 'AFM on' dashboard advisory light that will feature on production cars. During the Sydney-Melbourne run, without that light, the car didn't let on when AFM was engaged or otherwise. I made a point not to learn anything about how the system worked before the run because I wanted to see if you could feel it cutting in or out. Take it from me: on the Hume Highway, you can't. It feels exactly like a normal V8. Holden insiders say you can feel it cutting in and out - on a very smooth road. The Hume doesn't qualify.

David Eliot says there's a lot going on behind the scenes during AFM transitions, however. The first part of that involves turning the system on and off. Managing the torque on the transition into four-cylinder mode is done by controlling the throttle electronically. On the way back to V8 mode the spark is retarded briefly to cut torque quickly, and then the throttle is fed back in while the spark advances.

At the same time, the torque converter is busily managing NVH during the transition. It's basically a big fluid damper between the engine and the transmission, and it operates in three modes: open, closed and 'controlled slip'. Open is the way torque converters have been since Jesus played full back for the Nazareth Under-15s. It's good for NVH but bad for economy. Closed is the way a clutch in a manual is - extremely direct and locked up; bad for NVH but good for economy.

In controlled slip mode there's usually about 40rpm worth of rev difference across the torque converter (between the crankshaft and the transmission input shaft). That takes a bunch of harshness out, between the engine and the driveline. In the transitions to and from AFM, that slip goes as high as 120rpm to attenuate even more harshness.

This is why there's no AFM on manual V8s - the drive is too direct for the harshness inherent in switching between four- and eight-cylinder modes. It also explains why the biggest window for AFM operation is in sixth gear: it means higher road speeds, so there's more NVH from road irregularities and wind noise, etc. It masks the AFM transitions.

Eliot says Holden's biggest challenge was integrating AFM into the vehicle while covering up noise and vibration. "You have to put a lot of work into the engine, powertrain and exhaust mounts to manage vibration so you don't get booms in the cabin or vibrations in the steering wheel, pedals or shifter," he says. "It's almost as if we have to design it for two engines."

You derive the biggest benefit on the highway, with a light right foot. Highway speeds in sixth equate to about 1500rpm - the box seat for AFM operation. Eliot says, overall, AFM means about a one litre saving for every 100km you drive. "But on the highway, you'll do a lot better," he says. "Real-world driving is where it really comes into its own."

ADR fuel figures aren't the real world, clearly. They're derived from two lab tests. First is a city test that takes three minutes 15 seconds, and contributes 36.9 percent to the final (combined cycle) result.
Second is a highway test of six minutes 40 seconds and contributes 63.1 percent to the result. It means even the new ADR 81/02 'combined cycle' result is heavily biased towards highway running.

It's very interesting when you look at the cars returning nines and 10s in the ADR combined cycle fuel-consumption stakes. They include: Camry (117kW), Aurion (200kW), Liberty 2.5 (127kW), Maxima (170kW), Accord V6 (202kW), Commodore V6 (180kW) and Falcon 4.0 (195kW).

It's also very interesting to note some cars that, on paper, can't run from Sydney to Melbourne on one tank. They include: Yaris, Swift, Smart ForTwo, Micra, Colt, Mazda 2, Getz and Jazz.

On the highway, at least, it seems the 265kW AFM-equipped cars - Commodore, Sportwagon, Statesman, Caprice and Ute - allow you to have your cake ... and still overtake like you stole it.