Some vehicles come with either "all-wheel drive" (AWD) or "four-wheel drive" (4WD), and you may have wondered if there's any real difference between those terms. Cars only have four wheels, after all, so when "all" of them are doing the driving, that's four-wheel drive - isn't it? The logic makes sense, but AWD and 4WD have actually evolved into technical terms that refer to distinct mechanical systems. Whether you're shopping for a car or yours needs repairs, you'll want to take an educated approach, so let's walk through the ins and outs of each system.
Just the Facts If you've only got a minute and just need an easy way to tell the two apart, here you go.
AWD employs a front-wheel-drive layout but automatically redistributes power away from the wheels that are slipping to the wheels that have, and can use, the grip to generate traction. It's a relatively tame, set-it-and-forget-it system found in cars and car-based crossover SUVs (even though the latter are often marketed as 4WD vehicles).
4WD employs a rear-wheel-drive layout and typically includes a "dual-range transfer case" with a 4-Lo mode for low-speed off-road work. It's a relatively robust system found in trucks and traditional SUVs.
There are a few outlier vehicles that blur these boundaries – some have a transfer case but no low range, and others have simulated low-range gearing but no transfer case – but most systems you're looking at will fall into one camp or the other.
Want more details? Let's roll up our sleeves.
Drill-Down: Four-Wheel Drive Okay, so what exactly is a dual-range transfer case? In simplest terms, a transfer case is a device that "transfers" power from the transmission to both the front and rear axles, and the dual-range type includes low-range gearing (driver-selectable via a lever, dial, or button) that amplifies low-speed torque output for better traction on tough terrain. Virtually every off-road vehicle has one. Sometimes the default mode is rear-wheel drive, with both 4-Hi (for light-duty scenarios at higher speeds) and 4-Lo provided as optional modes. In other cases, "full-time 4WD" is standard, providing AWD-like four-wheel traction on paved roads along with the hardcore capability of 4-Lo on demand.
Drill-Down: All-Wheel Drive All-wheel-drive vehicles, on the other hand, are what you might call "full-time AWD." You can't turn the system on or off, and there's no low range; rather, it's always monitoring traction at all four wheels, ready to transfer power at a moment's notice in the event of a traction shortage. Every AWD vehicle has a default torque split, meaning the percentage of power that goes to the front and rear axles, respectively, when there's full traction all around. A common split is 90/10 on dry roads, with up to 50/50 available as conditions dictate. Some crossover SUVs even let you lock the split at 50/50 if you need to. But there's no rule of thumb here. What the systems have in common is their ability to re-route power from axle to axle (and frequently from side to side as well) when traction is compromised.
Which One's Better? It's depends on what you need. There's no doubt that 4WD vehicles dominate their AWD counterparts in more challenging off-road conditions. Low-range gearing enables the sort of rock-crawling, mud-flinging antics that would leave almost any AWD vehicle high and dry. Plus, 4WD trucks and SUVs tend to be equipped with a variety of beefed-up components that make them better-suited to off-road use.
But the downside is that all the tacked-on 4WD hardware brings added weight. More weight equals fewer MPG, an unnecessary burden for vehicles that have limited exposure to slippery or off-road environments.
Bottom Line There's no universally correct answer to this question. But if you're in the market for a new ride, we'd advise you to think about the type of driving you’ll normally do and go from there.
Thank you for visiting Hibshman's Auto Service, Inc in East Earl, PA. Count on our automotive repair technicians to keep your car, truck, suv, or van on the roads longer and safer.