Morris Minor Owners Club

To answer the original question.
I normally change out of 2nd at 20 and out of 3rd at 30.
If the car is moving at all or on a downhill slope I'll pull away in 2nd.
There is a saying that goes ' you buy horsepower but you drive torque' or something similar.

ah no wonder mine rattles a bit at 50 in 3rd

How is your blood pressure?
What sort of MPG do you get?
Are you 17?

I have the same 948 with stock gearbox, and the 4:22 diff and 14" wheels.
I've also checked my (modified) speedo against my GPS and it's accurate to within 4mph. I tend to change up at a max 4500rpm.

1st - 0-15
2nd - 15-35ish
3rd - 35 - 50
4th - 50+

There's also a little application on the web called "Gearcalc" which is interesting to play with.