Morris Minor Owners Club

There is only so much one can do with the available power. Depending on the power curve, the engine may not be able to attain its available performance potential easily - except in very favourable conditions.

13” wheels with low profile tyres might be an acceptable situation with the higher ratio diff?

I'd say no - it would be too high geared.

The standard Series MM diff ratio is 4.55:1 (same as a 948 Minor) and in my experience that is already plenty. It's also quite a bit taller geared than the Series II which is 5.375:1 - that has around the same top sped as the Series MM but is revving a lot higher. You can do the calculations, but with the standard 4.55:1 diff the engine isn't red-lining at its top speed, as it doesn't have enough power to get there, which suggests to me that it's a lack of power rather than revving out that limits its speed.

Also remember that the 4.22:1 diff is the Austin axle which is totally different to the Nuffield one in the Series MM, so it would not be a straightforward swap in any case.

I'm a little confused though - if you're talking about Series II cars, it's the 803 OHV engine rather than the sidevalve. The same would apply to that though - 4.55:1 would be as high as I would go (also having owned one in the past!).

M
I fitted a later axle with the 4.22:1 diff back in the 70's (and 8 inch front drums) to my MM and it seems 'ok' and happiest at about 55.
Cruising speeds were a lot lower in the old days!