Using spare tire on rear of car

I have 275-60-15 tires on the rear of my car and 205-70-15 on the front. I don't have a spare in the car but want to put one in it so I am going to get another 205 for a spare. I have read that running two different size tires on the rear could cause damage to the rear end if driven too far or too fast. Does anyone know if for example, I had to put that 205 on the rear if one of the rear tires went out, how far/how fast could I go without damaging the 8 3/4 Sure Grip?

I would not run different size tires with a locking differential myself.

I carry an electric air pump that plugs into the cigarette lighter. 99% of the time if you get a nail or screw in your tire they just leak down slowly so you can air it up and drive it to a tire store, or back home.

A few slow miles to get you home or to a repair shop would be OK, as it happened to me w/o noticeable damage to the 489 rear.
But just carrying some plugs/glue/tools 7 even a smaller pump would do better.
Sometimes putting on a spare tire is best/quickest, if for instance it's late night, bad weather, or even (cough-cough) a cheezy location.

A 205 and a 275 have a pretty big difference in diameter, but the 70 series against the 60 series could reduce the size differences by a lot as well.
I think for a spare, I would see if I could split the difference, maybe a 225 70 spare would work out OK on both the front and the rear. Then plan on only running the spare long enough to get the correct size tire back on the car.

To put things in proper perspective, a lot of people only have those doughnut size spares and they seem like about 1/2 the diameter of the normal tires. Those they recommend less then 60 miles and under 50 mph. Not sure I'd want to do a sure grip 8 3/4 that far, but a couple miles might be OK.

I am running P255/50R17's in the back of my 51. Which I picked because it's almost exactly the same height as the stock tires, but much wider. This way I can use a stock tire for a spare and my speedo is right. Granted I don't have a limited slip setup but I know the stock tire clears everything front or rear. I would not run two different sized tires on the rear with a limited slip for any longer than the least amount possible, it'll add wear. there is about 3/4" difference in height, don't care to do the math for circumference though.

A 205/70r15 has a 26.3" diameter
A 225/70r15 has a 27.4" diameter
And a 275/60r15 has a 28" diameter.


If the 225/70r15 will fit on the front and clear, that's what I'd say run as a spare.

offering another option I guess.
if you went with 2 space saver spares or a space saver spare the same size as the rears, then it would easily clear the front because they are only 4 inches across.
that way you don't have to worry about the rear diff.

I found the intrepid uses a 16 inch rim for it's spare and it clears the hubs correctly. you just need to order a new tire that fits the rim and is the same height as your rear tires.