Morris Minor Owners Club

If you find it is out by a margin, drive at a known 50mph (confirmed via GPS or what not) and get an assistant to place the needle at 50 (speedo glass off obviously). That should make it accurate enough for general use.

I broke my axle a couple of days ago, so I'm going through the same fix.
Does anyone have a guess as to why or how this happens?

Old age.
Due to the accelerating and braking of the car over time the axle half- shaft gets a rotational twist/set in it until it cannot twist anymore and then fails.
Lifting the inside wheel on cornering and the tyre then remaking contact with the road can give a short sharp shock to the half-shaft and which can again lead to failure.
Make sure that your axle casing is absolutely clean and there is no half-shaft residue left inside the casing. May be better to take both hubs off to ensure that cleanliness. I assume that the diff assy has been removed to get the broken half-shaft stub removed. Again that must be spotless before refitting.
Half-shafts get a rotational set/twist into them and if fitting a S/H half-shaft the S/H item must be scourced from the same side i.e. S/H drivers side half-shaft must only be fitted to the drivers side. If not the rotational set/twist will try to undo and the half-shaft will fail in very short time.
Upgraded half-shafts are a bonus but not all upgraded half-shafts meet the quality requirements so be selective if you go down the upgrade route.

Many of them have lasted 60 years, but if you drive hard enough they are more likely to break faster.

Axle tramp, dropping the clutch, speed bumps and pot holes all add a shock load.

I've got an Ebay half shaft coming. I guess it's 50/50 that it's the proper side.

The S/H half-shaft should be clearly identified with the side of the vehicle/axle that the half-shaft was removed from.
I would not hesitate purchase a second - side known - half-shaft because if the half-shaft you have purchased is the wrong side I doubt that it will last long.

The half shaft arrived without left/right markings. That's ok, because it's about 2 inches too short.
It's a BTA 806, for sprites and midgets. I guess they're a bit narrower cars. I'm back to searching for a ATA7057. Never too old to learn.

Having had three halfshaft failures now (over the course of the 28 years and 80,000 miles I've had the car), on long journeys such as to the MMOC National this weekend, I took a spare with me just in case...

For checking which side you've got, there is normally more wear on one side of the splines than the other. One side polished, the other side more dark.

That's interesting to know, as I recently purchased a couple of spares going cheap, just in case...