I don't think it can affect much if you drive at low speeds, I've seen some AWD with those silly space saver tires.SQOTD - what happens if you use 275/65/18 tires with a spare of OEM size 285/60 18 (Wouldn't be driving on the spare more than the distance needed to get to a tire shop, say 25 miles max) ? Probably about a 2% difference in circumference.