The ‘kid’ at the local tire place (Nat’l chain), will not sell me only 2 tires for my '97 RAV-4, stating that I need to buy 4 at a time because the AWD system demands it. I don’t drive around with the 4WD system locked in place, (I’ve used the 4WD button 2 times for a total of about 25 feet in the 11 years I’ve owned the car). So why would only buying 2 new tires be a problem? Or, is this kid just trying to make more $ ?
I don’t know how the drive system on a '97 RAV compares to that of an '08 RAV, but I know that the Owner’s Manual for the '08 model states essentially the same thing that you were told by the tire sales person.
On the newer RAVs, the 4WD button is for temporary low-speed locking of the differential–for situations such as getting out of a ditch–and when you push that button, it is a 4WD vehicle in a sense. At all other times, the car operates in AWD mode. So, even without pushing the 4WD button, the vehicle is an AWD vehicle, and thus, is subject to problems with the center differential if tires of varying circumferences are used.
Incidentally, if the tires are not worn evenly, that would seem to indicate that they have not been rotated as per Toyota’s maintenance schedule, and that in itself is problematic for AWD vehicles.
But, in any event, check the wording in your Owner’s Manual for the official word on this situation.
I think back then, Honda had what was called “Real Time AWD”, which was essentially FWD until wheelspin was detected. Some power is then transferred to the rear wheels. These systems are not quite as sensitive to tire diameter differences as “real” AWD, but as VDC pointed out, don’t take our word for it. Check the owner’s manual.