this post was submitted on 26 Aug 2026
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me_irl
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In addition to the previous answers, AWD and 4WD systems have important technical differences. A 4WD system uses a gearbox to mechanically lock the front and rear driveshafts together, such that they always have to spin at the same speed. AWD systems use a viscous clutch, allowing the driveshafts to turn at different speeds when some tires lose grip, at the cost of some wear on that friction surface.
That's why replacing the tires evenly is important on AWD, but not necessarily 4WD: different diameter tires spin at different speeds forcing the driveshafts to turn at different speeds all the time, wearing out that clutch prematurely.
Edit for completeness: on a 4WD system with differently-sized tires, the wheels would still be forced to turn at the same speed which means the tires themselves would be where the slip and wear occurs. That's why 4WD systems aren't supposed to be used on paved roads: having that slip is no big deal on a loose surface, but doesn't work well when all four tires have grip. (This also applies when all four tires are the same size, when the differential is locked and you're making a turn. The wheel on the outside of the turn has to spin faster the inside wheel, but can't, and so the tires scrub instead.)
In my experience, that's rare. Most AWD systems that I've seen use an open differential with no viscous coupler or locker of any kind. The '94 - '97 Toyota Land Cruiser (w/ ABS) in the US market used a viscous coupler, and Toyota called it "full-time 4wd". It also locked the transfer case in low range. To my knowledge, this was a really unusual configuration. They discontinued the viscous coupler after that, instead switching to their ATRAC traction management that used individual application of brakes to prevent wheel spin. Most of the Jeep SUVs in the last ~30 years, to my knowledge, used an entirely open center differential in AWD models.
It seems like modern, CUV AWD systems are usually FWD with computer controlled engagement of rear tires.
AWD and 4WD are also marketing terms rather than regulated in any way. In general you're right, but to be safe, always check what type of transfer case you're actually dealing with. Some "4WD" cars these days are AWD with super clever electronical controls for off road usage.
I suppose these days there's also the case where some cars just have electric motors attached to separate [groups of] wheels with no shafts connecting between them at all and don't really even fit in either of the two previous categories.
The exact meaning of "4WD" seems to depend on manufacturer. Toyota's 4WD works fine on hard surfaces because the differentials are normally open, which makes it a lot more like AWD. The center differential needs to be locked for it to behave like your described.