...when you have different front and rear tire sizes? A rear-sized tire on the front will interfere, and a front-sized tire on the rear will make your limited-slip diff unhappy. How far can you drive a limited-slip with different diameter tires? Okay, that depends on the difference, but what's the best choice for a spare?

If I had that, I would find a modern space saver spare and get both sizes.
you won't take up as much room as a full size spare, and you would be good to go no matter which tire went out.
The concord, intrepid LH body cars use a 16 inch wheel for the spare, with the same bolt pattern.
I found a tire that mounts that is the same size as a 255 60 r15.
you would have to test it out to see what works for each size tire.
https://tiresize.com/calculator/

that is the most modern one I could find, that you can still get ahold of. basically early 2000.
most of the others that have the right bolt pattern have the wrong size registry hole size and wouldn't bolt up.
like hyundai uses the same bolt pattern, but the registry hold is way too small.
Early 2010 toyota's had the same issue.
I think I got it for like 60 bucks and the tire was 100 at discount.

If you have a suregrip/limited slip you want a tire that is the same diameter as your rear tire , it doesn't need to be wide , just the correct diameter.

Do people actually change flat tires themselves anymore ? Most use a cellphone and triple A ...

I do usually change my tires myself as I don`t trust others to get it done without damaging anything. Not what you were asking but I usually take a battery tire inflator, plug kit and / or a can of fix a flat if I go on extended trips.

a plug kit, inflator, can of fix-a-flat, cell phone, and AAA.

In 50 years of driving and several 100,000s of miles, I've used a spare tire 2 times (both on the same car!) That car has a mini spare (a regular tire & wheel, factory equipment) that was at least 2" shorter then the 4 regular tires. I don't carry any spare tires in 2 of my 3 vehicles (the car with a spare is the same car I've used the spare on, believe it or not). When its shot, I will not replace it (I haven't checked it in a while, its probably flat, but I'm not sure it would come down out of the hole its stuck in anymore).

Given the difference between the factory spare and the regular tire on most modern stuff, and the concept the spare is only suppose to get you someplace to get the tire repaired or replace, I doubt the difference in size is that big of a deal if you get the tire fixed as soon as you can. If the difference is big, drive slowly, or have it towed in.

Back in the 70s Challenger days, the inflatable spare was 2" - 3" shorter then the original tires on the car and the factory thought that was OK back then.

A lot of new cars in that situation with limited slip and rear tires that wont fit the front come with a compressor and fix a flat.
I don't want to jinx myself but the two times I've had a flat the first time I had a full size spare in the trunk and called CAA/AAA to change it since i'm already paying a member fee. The The second time I was able to get enough air in it with a 12V compressor to drive home and then get a garage to fix it. Now I like keeping 12V compressors in vehicles just in case. Old truck, in winter at night. No way i was going mess around with the spare especially mounted underneath.

64 Sport Fury with rear 275/60 15 (28" dia.) Picked up a new rim and tire from Discount Tire. 15" steel rim w/ 235/70 15 tire (28"). I tested on the front as well with good clearance. My guess is the rim is a trailer piece.

Spare, what's a spare: whistling
AKA aside, my toy cars have empty trunks, no flat tires in them that I can remember now
My truck, car trailer (it has three spares) and daily drivers have them:
I have had many 15 inch trailer tires fail on the road with my enclosed 24 Ft car trailer I hate 15 inch trailer tires the trailer weighs 8600 Lbs. with the old pump gas(heavy) Duster in it hook up to my truck, the tires are rated at 2800 Lbs. each
My next enclosed car trailer will have 16 inch rims and tires
I accidently hit a rock in the dark with the passenger side trailer tires once and took both the rims and tires out, hence 3 spares now

I have different front and back too, found a Ford spare that is exactly in the middle, size wise.
So it works front and back with less than 5% difference in size for either.

I use a "temporary" spare from a Grand Cherokee (mid '90's?). The tire size is a T155/90D16. I picked it up at the salvage yard for $15.
Diameter is pretty close to my rears (295/45-17 drag radials) and bolt pattern is 5 on 4.5".
It stows easily in the trunk with factory mount. I got a scissor jack at the junkyard too (can't use the bumper jack with 'glass' bumpers

My 51 has P255/45R17's in the rear and P215/75R15's in the front. I carry the same sized spare as the front tires as it's diameter is within acceptable tolerances for the rear as well. That way I know it clears everything.

You only need to match the diameter of the rears for the LSD to be happy. I am not sure how relevant using modern applications is for out older stuff. I understand that a lot of later "limited slips" are essentially the computer braking the spinning when and not a mechanical connection like our old SureGrips.

This is what I do except I don't have an inflator, I have a good hand pump like for bicycles. I know it sounds dumb but I've done it before. Takes me about ten minutes with a few breaks between to get plenty of air in a tire to get to repair shop. If I wanted/needed I could take thirty minutes or so and get thirty pounds or so in one.

if you have a spare that is the same height as the front tires, and the front tires are shorter than the rears, you could deflate the other rear tire to about the same radius as the spare, when the spare is used on the rear. (Now THAT'S a run on sentence!)

^^^ Uh, I don't think it works that way...

I've taken a few 2,000-mile trips without a spare...but my general rules are:
Not getting hit by the side of a road
Old-car bumper jacks & tire irons suck
Short distance on mis-matched OD is survivable
Rims on a couple of my vehicles can be sacrificed if necessary
Credit card & tow is the final back-up plan if I've painted myself into a corner

Good back up plan. I do have tow insurance on my State Farm and it's ridiculously cheap!!! Something like $30 a year, maybe even less! Last resort though. Hate waiting on a tow truck, then the whole process of towing, riding with the tow truck.

Have you guys seen the way most people drive these days? There ain't no way I'm changing a tire on the side of the road.

AAA ( $90 a year covers me and my wife in any vehicle we are riding in, weather its ours or someone else's). and a credit card. If you are in a dangerous place, tell them, (on the side of the road is a dangerous place as far as I'm concerned, most of the time) the tow truck arrives pretty fast. If I need a roll back, I tell them that up front as well.

Good luck getting a tow truck when you have no cell reception.

Thanks for the reply. Please tell me why that wouldn't work.

Most places I drive have adequate phone reception. Should I be in a place that has none, and I get a flat, I'll deal with that at that time. Guess I'd limp on down the shoulder on the rim, because I'd also have no idea where I could get the tire fixed anyway. I'm not one to worry much about things that might never happen. For the record, I've been places with poor, or no phone reception, but didn't get a flat tire there.