Country differences (the writer seems to be Australian), plus some subtle technical differences few people know or care about.

Suffice to say, in British parlance any vehicle with four driven wheels can be called a "4×4"

Yes this, very common here too, nobody says AWD.

This distinction (although I know it) has never made logical sense to me.

I only have four wheels. If all the wheels are driving, and I only have four wheels, then...

Again, I do know what the conventional difference is, but it's just that: convention. Unless you memorized the convention beforehand, there's really no way to decipher why 4WD/AWD has any meaningful distinction, or which name maps to which mechanism.

What’s the distinction?

AWD is for on-road use. Each tire rotates at its own speed.

4WD is for off-road use. All tires rotate at the same speed.

That's incorrect. To have "All tires rotate at the same speed." you need 3 locking mechanism, which very few vehicles do.

At this point, the distinction is gone, because car bards marketing.

(AI something lies)

I've thought through the "all tires rotate at the same speed" thing before and how it works mechanically (front differential, rear differential, transfer case) and I think in the "no differentials locked" condition the correct statement is: the sum of the front wheel speeds == the sum of the rear wheel speeds. So you can have your front wheels slip and the rear wheels can still get power, or you can have your rear wheels slip and the front wheels still get power, but if you have one wheel in the front and one wheel in the back that are slipping, you're hosed.

Re: the 3 locking mechanisms comment, in typical 4WD configurations the transfer case in 4H/4L is essentially locked (which is what makes the above sum front == sum rear condition). AWD transfer cases can be more flexible there; some essentially act as a 3rd differential and result in all four wheel speeds being essentially decoupled (for better or worse, easier to get stuck) while others actively control the front/back locking with clutches or hydraulics or whatever.

That's incorrect. To have "All tires rotate at the same speed." you need 3 locking mechanism, which very few vehicles do.

The "locking mechanism" known as traction works pretty well most of the time and has the nice side effect that it only wears an easily replaceable consumable when you break it.

All tires don't rotate at the same speed even in 2WD, otherwise you wouldn't be able to turn without slipping.

For offroad use, you want all wheels to be locked to each other. Otherwise one wheel without grip will allow all other wheels to remain unmoving. You're right that you don't want this behavior for onroad use, which is why the differentiation (and the differential) exists.

AWD is an automatic 4 wheel drive when required.

Think of it more like it's going to be 2WD most of the time, but when there is low traction (such as ice) the 4WD will engage.

4WD is a permanent thing, or at least always on when you engage it.

I don't think that's universally true

Indeed. Marketing ruins all definitions.

I don't think most people here have it right. From my understanding the main difference is that 4WD can be manually turned off (meaning going to 2WD mode). AWD is always on.

Some AWD systems might intelligently change power to different wheels (or front and back) and that might vary per brand and model. 4WD could also do this. Usually with 4WD you might also have a 4 lo and 4 hi gear as well.

Then there's also various types of differentials and locks and whatnot which is a whole more detailed discussion.

The RAV4 dynamically engages the back wheels with no mechanical coupling to the front wheels. It uses a separate electric motor in the back. Toyota calls it AWD.

[delayed]

Clutch pack vs transfer case.

It's more of an industry wide thing:AWD means all wheel drive even when the vehicle has more than 4 wheels. So in the case of a vehicle with more than 4 wheels, 4 wheel drive would indicate four of the wheels are driven and the remainder are idlers. For instance 6x4 would indicate six wheels with four driven wheels.

https://en.wikipedia.org/wiki/Six-wheel_drive

So nwheels x n driven wheels.

You seem to be talking about something different. The conversation is about vehicles with four wheels and the differences between 4WD and AWD in that context. Perhaps you're saying that there is no difference?

But 6x4 notation is handy when you have six wheels, sure.

With a four wheel vehicle there is no difference between 4WD and AWD, just like with a six wheel vehicle there is no difference between 6WD and AWD.

That’s definitely not accurate.

There is no official definition afaik.

My understanding:

AWD: All wheels can get power and differentials are used for turning, etc. Therefore, if one wheel has no traction (snow / mud / ice) and slips, it draws all the power and you can't move without restoring traction.

4WD means all wheels spin at equal rates; there are no differentials. Therefore even if one wheel slips, the other three will pull you forward. Great for off-road but sucks for higher speed driving with turns.

4WD: Distribution of power to the wheels is static. AWD: Distribution is dynamic.

4WD vehicles are often part-time 4WD, meaning they can be switched between 4WD and RWD or FWD.

AWD vehicles are full-time AWD because part-time AWD is unnecessary given that the drivetrain delivers power where it's most needed.

There are plenty of AWDs that operate as FWD 90% of the time, only engaging the rear as needed. (I am told this is for fuel efficiency but the logic escapes me.) But I have yet to ever see a 4WD that could be "switched" to FWD.

The clearer distinction is that a 4WD has a second gearbox (aka a transfer case) that either manually or automatically engages the front wheels, the Jeep Wrangler /TJ being the architype.

> There are plenty of AWDs that operate as FWD 90% of the time, only engaging the rear as needed. (I am told this is for fuel efficiency but the logic escapes me.)

AIUI, you lose energy because the power/torque has to go through an extra differential, transfer case, etc, before reaching the rear wheels.

Going from the engine through the transmission to the front wheels is already a 'fixed cost', but the extra mechanics (and their friction loses) to the rear wheels an 'OpEx' that you don't necessarily have to pay if you don't need to.

>> the extra mechanics (and their friction loses) to the rear wheels an 'OpEx' that you don't necessarily have to pay

True for a FWD v RWD car decisions, but in an AWD are not the rear differential and driveshaft still turning whenever the rear wheels are turning? Even the clutch packs should still be rotating even if disengaged. The fact that there is no power through them doesn't mean they aren't generating losses.

It depends. With conventional AWD (like all Subarus), you are correct that there is significant energy loss due to the AWD because there is a drivetrain to the rear wheels, even if it is not delivering power. Many hybrids, however, have AWD and they work by having a second, small electric motor that powers the rear wheels only when needed. That means no physical drivetrain to the rear wheels, so no energy loss. That's why a hybrid Toyota RAV4 AWD gets about 15% better gas mileage than the smaller and lighter hybrid Subaru Crosstrek AWD. The disadvantage of the electric motor approach is that it cannot deliver as much power to the rear wheels as a conventional physical AWD drivetrain can. That said, probably only an issue in deep mud or snow.

Well, no physical drivetrain to the front half of the car would make a difference. But even with electric motors in the rear there is a drivetrain between the wheel and the motor. If the idle rear electric motors are is still spinning then the total mass of spinning parts might well approach the mass of a Subaru drivetrain. Driveshafts are very light.

It sounds like the savings are more in construction costs than fuel savings.

It's more to do with avoiding drivetrain binding than fuel efficiency (although the two are related). When you corner, if all four wheels are driving at the same speed, then you'll end up forcing wheels to rotate faster or slower than they need to go. As a rule, when you corner, your front wheels travel further than the back wheels.

If your back wheels are rotating at the same speed as the front wheels while you turn, then you'll induce wheelspin on the rear and potentially lose control, or damage your tyres, drivetrain, or engine.

Full-time 4WD and AWD cars have three diffs, one on each axle, and one between the two.

Others have mentioned the RAV4 which actually has two separate drivetrains, which is a different implementation all together.

most AWD (mostly FWD) vehicle's engines are tranvserse mounted (horizontal in engine bay). These vehicles usually do not have a traditional transfer case/differential liek a 4wd vehicle, but instead a sort of "power siphon" that puts power to rear wheels selectively.

Most RWD and 4wd vehicles are longitudally mounted, allowing for easier packaging of power to the rear of the car.

The transverse packaging works much better in FWD configuration as

1. Packaging; the relation of the "AWD" (mostly FWD) transmission and a tranvserve engine is naturally a smaller form factor than other combos. Plus, the lack of traditional diff/transfer case is tons of weight savings.

2. Average driving; "FWD mode" in AWD vehicle is functionally equivilent to a FWD vehicle. This is ideal for most driving, IE; snow, fuel economy, understeer etc.

It's all marketing wank with overlapping venn diagrams. Nobody for whom the difference matters engages in hair splitting over language.

4wd - locking transfer case in the middle. Best for off road.

AWD - clutch packs. Best for nasty conditions on road, IMHO.

^ this. 4wd is used on offroads and locks all wheels together, transfering equal power to all wheels. AWD varies power using different technologies, from a simple Haldex that transfers power back when front wheels slip to complex sensor controlled individual wheels. Some are front-biased (generally but not exclusively cheaper ones), some are rear-biased (mainly in sports cars) and some are true AWD, depending on the needs and expected pricing of the vehicle.

Not quite, no. It only locks all the wheels together if you also have axle lockers.

You might have something like a Series Landrover transfer box where in 4WD mode the front and rear axles are locked together and in 2WD mode only the rear axle is driven.

But does that make a Defender, RRC, or Disco with LT230 transfer box which has a lockable diff AWD or 4WD? The centre diff supplies power to both front and rear all the time, but can be locked in slippy conditions otherwise if one wheel spins *none* of the wheels get drive - the axle and centre diff will both spin.

Then what about the later Borg-Warner transfer box in later Range Rovers, where you still have a centre diff and epicyclic gearbox to set high and low range, but there's no diff lock - there's a massive viscous coupling that turns it into a ridiculously stiff LSD.

I doubt that there was ever a “legal” definition of AWD vs 4WD.

But in modern times, technology has most definitely blurred the distinction. Higher-end Jeeps also have a “full-time 4WD” that sounds an awful lot like an AWD system that can also do 4WD stuff.

So, eh, it’s probably not a good idea to argue too much about broad categories anymore. Individual systems must be compared

My first guess was a RAV4 as well from that description