Car buyer's guide: The right gear at the right time

The Subaru Outback is an all-wheel-drive. Photo / Supplied

You might think that four-wheel-drive (4WD) and all-wheel-drive (AWD) would be the same thing but, as George Gershwin once noted, it ain't necessarily so.

Four-wheel-drive has come to mean a vehicle with low-range gearing. When low range is chosen, drive goes to the wheels via another set of gears that multiplies by double or more the "pulling power" of each normal ratio.

In first-gear low range, the vehicle crawls along at a walking pace and has power to get a loaded trailer moving or to drag heavy objects.

Low-range gearing also offers the driver better control in challenging conditions, such as off-road tracks and trails. But because the gearing is so much lower, the vehicle's top speed will be cut significantly.

Many want the advantages of having all wheels driven when launching a boat, driving to ski fields or on wet, slippery suburban roads.

The other big advantage is in performance and motorsport vehicles, where 4WD helps to get vast amounts of torque to the road efficiently and enhances roadholding.

So manufacturers responded by offering 4WD without low-range gearing. These vehicles became known as all-wheel-drives, pioneered in the 1980s by the Audi Quattro.

You can usually assume that something described as an AWD won't have a low range. Sort-of exceptions are manual Subaru AWDs with "split-range" gearing. This provides a small reduction, but nowhere near the ratios of a conventional low range.

But we're not quite sorted yet. A 4WD may not be a 4WD at all, unless the driver pushes a lever or turns a knob to engage the front wheels. There are full-time and part-time systems.

Full-time 4WD means that drive is going to all wheels all the time, as long as they all have traction. Part-time 4WDs are usually rear-drives on which torque can be sent to the front wheels whenever the driver engages them.

It's common on most utes and some SUVs. On a few vehicles, such as the Toyota Land Cruiser 70 Series, the driver has to get out and lock the front hubs to complete the drive train.

Part-time 4WD is cheaper to make and has a small fuel-economy advantage when only the rear wheels are engaged. Some AWDs also provide full-time drive to all wheels; others are basically front-wheel-drives that send squirts of drive aft when sensors detect slippage.

A word of caution if you're shopping: some AWDs come in 2WD versions. These became popular because buyers reckoned there was no point hauling around extra weight and complication - and paying more - if they thought they wouldn't need it.