Hello Friends!
I'd been looking for a full size spare for my 240 wagon to replace the mysteriously exploded donut. On a recent junkyard visit I found a like new wheel/tire from a sedan. It is 18570R14. The tires I have mounted on my wagon are 19570R14. According to a tire size calculator, the spare is about 0.5" smaller in diameter and 0.4" smaller in width.

I know for safety reasons it isn't the right thing to have a mismatched tire. Since this is a spare and will likely be used only temporarily and for short distances I am OK with this risk. What I am not OK with is potentially damaging the suspension/drive train.

So the question is will having a 19570R14 and 18570R14 on opposite sides of the front or rear axle cause any damage when used temporarily?

If the new spare is not the same diameter as your other tires, it could give problems if you have ABS or limited slip if mounted on the rear. But do you know if the original donut was the same diameter as your other tires? Probably not. A spare is only for emergency use. A full size spare the same as your other tires can be rotated in sequence with the others to equalize wear.
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1992 745, >500k km (now gone, but not forgotten)

original poster wrote - "I'd been looking for a full size spare for my 240 wagon to replace the mysteriously exploded donut"
A couple of things to consider--The space saver spares deteriorate faster than street tires---and I'd carefully look at the muffler to tailpipe connection. A slight exhaust leak there may not be noticeable but can cook a spare. -- Dave

Good point about the muffler connection. I'll take a look at that. I may mount it up and take a drive around the block to see if it affects the ABS sensor. Appreciate the responses!

No it will not damage anything

If the 92 has ABS, I'm just curious what the mismatch in speed might do as the ECU is trying to process and/or adjust for why one wheel is turning faster than another. Don't know if it's an issue -- just a thought that occurred to me. I wouldn't want to go a long way with the mismatch on the rear - but mechanically for short distances I wouldn't be concerned.

IIRC, the 240 ABS system used the speed sensor on the differential as input, and if so, the slight difference in individual rear wheel RPM's with mismatched tire sizes should not matter. I believe this was called 3-Channel ABS, whereas 4-channel ABS monitors each wheel separately.

The way to be certain on your car is to look at the rear wheel brake drum/disc assemblies and see if there is a tone ring (toothed wheel) attached to each, with a little sensor and wiring close to it.

If not, it shouldn't upset the ABS with mismatched rear tires. As Michael Yount said, it's not advisable to go a long distance with that setup, as the diff gears are "working" all the time, instead of only when rounding a curve.

Finally, the 245 wagons used somewhat larger tires than the sedans...did Volvo supply a correspondingly larger spare tire to exactly match the OE wagon tires? If not, I would think they found that small differences in tire size do not cause any ABS confusion.
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Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), and XC60. Also '77 MGB and four old motorcycles. Long gone: 1981 244, 1994 940 and 1998 S90.