It shouldn't matter either way in the short term. Driving with the small spare tire doesn't cause any immediate damage, though I'm sure it's no recommended in the long term.
For 2WD cars, having different diameter tires on the front than on the back won't matter a bit either. Side to side (or any diameter for AWD) might wear the coupling mechanism in an unwanted way, but I'd assume it wouldn't matter unless you did it for 100,000 miles or something :-P
If you have a "limited slip" differential, mismatched tires on that axle will wear out the clutches prematurely.
It isn't going to hurt the transmission. The vehicle might not drive right with mismatched tires.
On the same axle? It will have a negative effect on the diff, and mess with the cars handling. So its a bad idea. Slightly different tires on different axles isn't so bad.
Well I originally had p225 70r 15 tires on the truck. 3 of the tires are p225 75r 15. Should the 4th tire be p225 70r15 or p22575r15?
Yes, if they are different in respect to each other on the same car. Some of the newer all wheel drive vehicles are very sensitive to this.
Mismatched tires on the front end ruins alignment and eats-up the limited slip on the rear.
No.
If you turn a corner one wheel is turning faster than the other. The differential is built to deal with it.
it is unsafe to drive with non-matching tire sizes
