What's the deal these days with radial ply tires? Are you allowed to switch sides or not? For the first time in years I have a car with a full sized spare and the manual says to include it in the rotation on one side only. That seems kind of dumb as those 3 tires will be have that much less wear than on the other side where the rotation is just swapping front and rear.

Is it still true that you should not swap sides with radial ply tires? I'd like to include the tire in the rotation because if it could be rotated freely it would extend overall tire life by 25% but if it can only go on one side (not counting actual use as an emergency spare of course) then maybe not.

I guess I could leave it out of the rotation and when the 4 on the car wear out I could then buy just 3 (assuming the identical tires are still made) and utilize the spare at that time, and put aside a worn one as a spare. But using an old tire, even an unworn one, may not be that safe an idea either.

So what's the scoop? Is the no-switching-sides thing just an old wives tale that's been superceded?