Yes all the wheels are measured for the ABS and tyre pressure system, but not for the 2wd/4wd system switch, unless some percentage of drive is being given to the rear (rare). If you had a space saver that had a slightly different diameter on the rear you may get a tyre pressure warning and possibly an ABS warning but it wouldn't harm the drive train. If you had it on the front then the system would be constantly adjusting the amount of rear drive to try to make up for the supposedly slipping front wheel but it wouldn't be able to correct it as there isn't really any slipping. I remember when the BMW X3 first came out there were a lot of transmission problems caused by slighly differently worn tyres or even tread patterns as these cars are perm 4wd.