After buying my 2020 Forester I was told that the tires should only be replaced in sets of four in order to keep them all about the same diameter. Or if a single tire has to be replaced, then only do so with a tire that has been shaved to match the other three.
My understanding of this is that it minimizes the work that one of the diffs (central/transfer?) has to do when compensating for different sized tires, and that this extra work is detrimental to the diff due to overheating if the car is driven like this for an extended period to time. The answers to this AWD tire replacement question seems to support my understanding of the issue
Which brings me to the question of the donut spare. This spare is significantly smaller than the normal driving tires. Thus it should affect the drive systems a lot more than simply replacing a single road tire. This donut tire drive distance question was framed for a 2WD car and mentions some ball park numbers for how far you should drive on a donut.
But do these numbers relate to an AWD car like a Forester? Or if not, what would be distance numbers for an AWD vehicle?
(Part of the rational for this question is that I sometimes drive on dirt roads in the middle of nowhere in the SW of the US and can be a long way from civilization. I want to understand if I should invest in a cheap, full sized spare for those times when I inevitably get a puncture like I did yesterday - fortunately this time I was only 10 minutes from home when it happened, after driving for 7 hours on various back roads)
FWIW my main tires are 225/60R17 and the donut is 145/80D17 (both are the stock tires that came with the car). This means the Subaru supplied donut is 1.5" smaller in diameter than the main road tires.
Given that my normal tires have been worn down a bit, this does decrease the impact of the (hardly used) donut. And of course if I buy a cheap full size spare, there will always be a difference in diameter between that and the current state of the normal tires.
Although I was initially interested for AWD vehicles in general, here is what my manual says for my 2020 Forester
9-3 Temporary spare tire
The temporary tire is smaller and lighter than a conventional tire and is designed for emergency use only. Remove the temporary spare tire and re-install the conventional tire as soon as possible because the spare tire is designed only for temporary use.
Check the inflation pressure of the temporary spare tire periodically to keep the tire ready for use. For correct tire pressure refer to "Temporary spare tires" (Page 499)
When using the temporary spare tire, note the following.
Drive with caution when the temporary spare tire is installed. Avoid hard acceleration and braking, or fast cornering, as control of the vehicle may be lost.
Do not exceed 50 mph (80km/h)
Do not put a tire chain on the temporary spare tire. Because of the smaller tire size, a tire chain will not fit properly.
Do not use two or more temporary spare tires at the same time.
Do not drive over obstacles. The tire has a smaller diameter, so road clearance is reduced.
When the wear indicator appears on the tread, replace the tire.
The temporary spare tire must only be used on a rear wheel. If a front wheel tire gets punctured, replace the wheel with a rear wheel and install the temporary spare tire in place of the removed rear wheel.
There is nothing mentioned about how long you can use the donut, and the only real limitation is not to go over 50 mph. And this seems to totally go against the requirement that all 4 normal tires need to be matched for size.
Note that the donut itself doesn't have any more info stamped on it that what I have already provided above.