Tires: Tire size - spare, tire diameters, rim size

Question
I have always been told that a spare should never be of a different tire than the other three on the vehicle.  Radial/bias, tread, or size.  New vehicles (Chevy) are being delivered with 17" main tires and spare tires that are 16". They are "full size" spares (not donuts) P245/65R, so it is conceivable the driver can/will drive on this spare for more than "emergency" means.  IS this not unsafe, and especially  damaging to the diff. - and/or transfer cases on AWD cars?  When did this practice start?  It is a surprise to me.

Answer
Bob,

Here's the rule:  All tires ON THE GROUND are supposed to be the same, with the exception of the temporary use of the spare.

The reason for this is 2 fold:

1)  So the vehicle handles properly.  An odd tire tends to make the vehicle pivot around it in emergency manuvers, just when you'd want it to behave in a straight and true manner.

2) So the tire diameters are the same, so the differential isn't overworked.  In FWD and RWD, an odd sized spare isn't much of a problem unless the temporary spare is mounted on a drive axle.  But the vehicle manufacturers always design the overall diameter of the tire (the rim size is unimportant)so that the differential can accommodate the temporary use of the spare.  In the case of 4X4's and AWD, the spares are usually the same as the ground tires, but sometimes they aren't.  Sometimes they are different, but the diameter is very close.  And usually it is obvious that this is a temporary spare because the rim is an ugly black steel, where the other rims are aluminum.

But with the advent of plus sizing (larger rim diameters) many vehicle manufacturers are using the "base" tire size as the spare and sizing the optional tires so they are the same overall diameter. That eliminates the problem for 4X4's and AWD.