Most of the time, the only way to tell what is propelling your BMW Active Hybrid 5 in heavy traffic is to glance at the rev-counter. If you're doing less than 60km/h and not actually accelerating, chances are it's the 40kW electric motor, and the rev-counter needle is stationary on zero - which is just as disconcerting as it sounds.
As soon as you accelerate even slightly, however, the needle jumps up to about 1500rpm (you don't feel or hear anything at all) and the stately Hybrid 5 proceeds on its way in plutocratic near-silence, gathering momentum while all around you lesser vehicles rev and spit and change gears and rev again, sounding just so 20th century.
Elitist? Yes. Revolutionary? Not really.
As with most hybrids, the first BMW petrol-electric to be released in South Africa demonstrates its green credentials best in an urban environment, which is why the first section of the launch drive took us through an industrial area north of Cape Town on a Friday mid-morning.
ANY TIME IT THINKS IT CAN GET AWAY WITH IT
And the cleverest part of the whole deal is that BMW has used the flexibility of the electric drive to take the idle-stop concept to the next level.
Whereas most idle-stop systems will only switch the engine off when the car is standing still, in neutral, with the brakes on, the Active Hybrid 5 will shut down the three-litre straight-six any time it thinks it can get away with it. And if you keep one eye on the rev-counter, it's astonishing just how often that is.
Even coasting downhill at close to the national speed limit, the engine will sometimes switch off, while the power diagram on the dashboard's central touchscreen shows the electric motor generating “engine braking” by converting momentum into amps, boosting both the car's batteries with energy that would normally be wasted as heat.
But when you touch the accelerator pedal, or even when the car comes to the bottom of the hill, the engine is there as needed, providing the superbly polished performance we have come to expect from BMW's big sixes.
POISE, PANACHE AND POWER
It's the same around the twisties; the extra weight of the electric motor and the lithium-ion battery pack (75kg in all) doesn't seem to affect the Hybrid 5's road manners at all; it disposes of corners with all the poise, all the panache and all the power of its conventional siblings.
A flick or two of the paddle shift going in, to get the revs up, and a bootful of loud pedal coming out will reward you with superbly modulated power and steering, a minimum of body roll and utter disdain for bumps and ridges in awkward places, all accompanied by that familiar six-cylinder BMW snarl, suitably restrained in this iteration as befits an executive carriage.
The only time the added complexity of the ActiveHybrid system acts against the car's best interests is when you ask it unexpectedly for full-bore acceleration.
Mash your foot flat at the lights and the car will move forward slowly for a split-second before the petrol engine wakes up with a distinct jerk and the car begins to accelerate properly.
Similarly, if you pull out from behind a slow-moving vehicle and try to overtake, the eight-speed auto transmission will hesitate for a heartbeat before first calling on the electric motor for back-up, then changing down, beginning to accelerate, and then changing down again for some real voomah.
All of which is just as nerve-wracking as it sounds.
The answer is, of course, to remember that the Active Hybrid 5's active ingredient is a high-revving three-litre turbocharged engine, and to treat it like a manual: hit the left-side paddle twice to change down before you pull out to overtake, making sure you have at least 4000rpm on the clock, and you'll have all the acceleration you dare ask for from a nearly-two-tonne executive sedan.
SO, HOW DOES IT WORK, THEN?
The internal-combustion side of the equation is BMW's renowned three-litre straight-six - twice World Engine of the Year - with TwinPower turbocharging and direct fuel-injection, delivering 225kW at 5800rpm and 400Nm from 1200-5000rpm, exactly as found in the 535i.
The electric side is a 40kW large-diameter synchronous 'platter' motor in the bell-housing of the eight-speed Steptronic transmission, roughly where the torque convertor would usually be, giving a combined output of 250kW and 450Nm.
It's powered by a 316-volt lithium-ion battery pack whose 96 cells have a combined capacity of 675 watt-hours. The 47kg battery pack sits behind the rear seats and between the rear wheels, well protected from accident damage, but reducing boot capacity from 520 litres to 375.
BMW says the electric motor alone will take the car up to four kilometres at up to 60km/h, but in practice the petrol engine always seems to kick in at about 40km/h, or as soon as the car hits the slightest incline.
A normal 12-volt battery and starter motor are used for starting from cold, but the idle-stop system uses a smoother-acting generator/motor that charges the 12V battery (mostly on the overrun) and provides those almost-imperceptible starts in traffic.
The rest is down to some very sophisticated ECU software and one of the silkiest automatic transmissions in the business.
COMPARISONS ARE ODIOUS
BMW has come late to the hybrid arena and, rather than start a new line of hybrid models as Toyota did with the Prius, it has chosen to 'green' existing models by adding an electric motor and a high-voltage battery to the package. And, being BMW, the package is seamless, sophisticated and very muscular.
Comparisons, they say, are odious, but this one is perhaps inevitable.
Compared to the conventional 535i, the Active Hybrid 5 is 75kg heavier but, thanks to the extra power of the electric motor, its performance is identical: 0-100 in 5.9 seconds, 250km/h (limited) flat out.
The Hybrid 5 costs R760 976.50 including CO2 tax, the 535i, R684 356.50.
But here's the kicker: the Hybrid 5 uses just 6.4 litres per 100km in the combined cycle, where the 535i burns through 7.9.
Is a 19 percent reduction in fuel consumption - and greenhouse gases - worth the extra weight, the extra complexity and the extra R76 620? That's your decision.