Yeah a full size spare is better, but I hope you wont have any problems. My first question is how many miles did the tires have on them? if you have 2 new and 2 old, the TOD is probably acting up because of the different diameters and how they affect the TOD sensors associated with wheel speed. Depending on that, it may be wise to replace the other two as well. You cant go wrong with doing all four if possible although the cost is hell. For a quick test, slightly air down the new ones and measure the height of all the tires until they are the same height. Dont worry about what pressure they are- just the height. Make sure you drive in a controlled area. If the TOD system seems to run smoother, them it is the tire diameter and you will need all four the same height. Hard handling, acceleration, and braking wont tell you too much because the lower aired down tires will flex more and the diameters will vary with the less pressure. Dont forget to air them up properly!!! Second question is are the new on the front or rear?