You’ve probably seen the term four-wheel-drive – usually abbreviated to 4WD or 4x4 – applied to many cars, particularly big SUVs. It refers to the fact that the power produced by the car’s engine is distributed between all four wheels, rather than just the front or rear wheels.
It’s not to be confused with the much more common all-wheel-drive (AWD), which works differently. It may not be as prevalent as it once was, but 4WD is still the best solution if you want to climb mountains, ford streams and tow heavy loads.
In this guide we’re going to explain how 4WD works, how it’s different to AWD, and what the benefits and costs of having a 4WD car are.
How does 4WD work?
First, we need to establish that you’ll only find a 4WD system of the sort described here in cars with an internal combustion engine. There are electric cars with four driven wheels, but that’s achieved with multiple electric motors that aren’t physically connected to each other.
Now, in a two-wheel-drive car, the engine’s power is channelled through the gearbox to the front or rear wheels. In the latter case, the gearbox spins a shaft that connects to the back axle.
In a 4WD car, the gearbox transmits the engine’s power via a shaft to a transfer box, which splits the power (via more shafts) between the front and rear axles. In the crudest 4WD systems, the power is split equally between the four wheels, but the latest electronically controlled systems can vary the split according to the prevailing conditions and/or which drive mode is selected.
There are two variations on 4WD: permanent and part-time. As the name suggests, permanent 4WD – found in cars such as the Land Rover Discovery – constantly sends power to the car’s four corners. A part-time system – found in the SsangYong Rexton, among others – defaults to rear-wheel-drive until the driver selects 4WD using a lever or button.
How is it different to AWD (all-wheel-drive)?
A car with all-wheel-drive is essentially front- or rear-wheel-drive but has the ability to send some (or all) of the engine’s power to the opposite set of wheels under certain circumstances. In contrast, a 4WD system has the ability to send power to all four wheels all of the time.
We have a full guide to all-wheel-drive, but here’s a flavour of how it works. AWD cars distribute power to all four wheels only when extra traction or grip is needed. If the main drive wheels start to spin, power is diverted to a central differential which then transfers as much power as is needed to another wheel, stopping the spinning. Some AWD systems are entirely mechanical, others are electronically controlled.
It’s an immensely complicated solution, but much more fuel efficient that 4WD. That’s because it’s a more compact, lighter system, and only powering two wheels most of the time puts less strain on the engine.
What’s the difference between a 4x4 and a 4WD?
Nothing, essentially. 4x4 is simply another way of expressing 4WD, indicating that a vehicle has four wheels and four-wheel-drive. A front- or rear-wheel-drive car could be described as a 4×2 using the same format. These days, it’s more commonly used in reference to lorries – would you believe there’s such a thing as a 12×12?
4x4 had another use in the past. It used to be a catch-all term for the kind of car we now call an SUV – Land Rovers used to be marketed as ‘the best 4x4xfar’. However, not all such cars actually have 4WD anymore, so the term has fallen from favour.
What are the advantages of 4WD?
The only part of a car that’s constantly in contact with the road is a very small area of the tyres’ upper surface. And all of the engine’s power has to be transferred to the road via that tiny ‘contact patch’. In a two-wheel-drive car, the tyres have to deal with 50% of the power. That’s difficult enough but add a slippery surface to the equation and the job can become virtually impossible. The wheels lose traction, spin up and the car goes nowhere fast.
With 4WD, each tyre only has to transfer 25% of the engine’s power to the ground. That means they have more traction and are much less likely to spin up on a slippery surface. That’s why off-roading SUVs have 4WD. Conditions can be extremely slippery on wet, muddy trails but the extra traction a 4WD system generates keeps the car moving. The extra traction is also beneficial if you need to tow a heavy trailer.
What are the disadvantages of 4WD?
A 4WD system is bulky and heavy, which has all sorts of knock-on effects that are less than ideal. For instance, the 4WD system is entirely packaged under the car’s floor in most SUVs, which is part of the reason they’re so tall. A tall car is less fuel efficient and doesn’t handle as well on the road as a lower car. If the designers want to produce a lower car, the 4WD system takes space out of the interior.
The extra weight of a 4WD system also impacts fuel economy. And the engine has to work a bit harder to overcome the extra friction generated by all the cogs and shafts in the system, worsening economy further. The extra components add cost to the car, as well, so a 4WD model will always cost more than the two-wheel-drive equivalent.
How often should you engage 4WD?
Cars with part-time four-wheel-drive or all-wheel-drive allow you to lock them in 4WD mode, via a button or lever. That should really only be done when absolutely necessary, for instance when you’re faced with a snowy hill or a muddy trail. Using 4WD when driving normally will increase fuel economy and potentially wear out the car’s transmission. You might also find the car can’t turn quite as tightly as it normally would.
What sort of cars have 4WD?
There used to be all sorts of cars that had four-wheel-drive – everything from the tiny Fiat Panda 4x4 to the Porsche 911 Turbo supercar, via innumerable run-of-the-mill family cars. Now, though, the vast majority of cars that can send power to all four wheel actually use an all-wheel-drive system, because it’s more fuel efficient. The relatively few remaining cars that have a proper 4WD system are pretty much all big SUV, such as the Toyota Land Cruiser, Land Rover Defender and Volkswagen Touareg.
Do you need 4WD in the UK?
If your lifestyle involves a lot of off-roading and towing heavy trailers, four-wheel-drive will undoubtedly make driving easier. For everyone else, though, the necessity of having 4WD in the UK is debateable.
It can certainly be tempting to get a 4WD car if, for instance, you live in an area that’s prone to harsh winters. If that involves deep snow, you probably do need a 4WD car with lots of ground clearance. But, if you’re just tackling ice and compacted snow, a two-wheel-drive car fitted with winter tyres will usually cope perfectly well. Go to any European mountain range and you’ll see that’s what most people drive.
Looking for more jargon-busting motoring meanings? Head over to our Parkers Car Glossary page and take a look at our other definitions.
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