What effect Would a donut spare have on a differential?

[mrpj]

Originally Posted by Bluevitz-rs

With a regular open diff it will no effect. You should not if you have a limited slip diff.

However as others have said, you really shouldn't ever use a space saver on the front as 75%ish of your stopping power comes from the two front wheel. As well as steering.

I agree -
But if you are some distance from the nearest place where the flat tyre can be repaired or replaced, or home for access to another jack (if owned)or some means of supporting the car and only have the vehicle jack, there really is no other option. How many of us carry around a trolley jack, axle stands etc.?
Hence, I would suspect the 50mph limit to allow for the smaller width tyre.
I have been in that situation and, if driven sensibly, it does not pose a problem.
e.g. take corners much slower, think ahead, remain a longer distance from vehicles in front etc.