Last year we had a flat 100 miles from home and seeing the 50 on the spare the driver on the day assumed it was kph. They were a lot later than 12 minutes.
Perhaps the 'use once only' refers to the fact that some handbooks now put a cap on the spacesaver mileage of 3000 miles typically. That's a lot of screws.
Afaik one of the reasons for the speed limit is to reduce the work differentials have to do especially 4wd vehicles.
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