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Due to a very fortunate encounter today I now have a 1275 engine and ‘box from an ADO16.
My quandary is IF I use it what gearbox/diff ratios to use and how far to tune it ?
It is on 10 inch wheels and is primarily a road car maybe with the odd sprint, hill climb or tour now and again.
Your guidance and wisdom greatly received.

It might depend on whether you're doing a lot of motorway driving, town driving, or spirited B-road driving (or perhaps a combination thereof). Others will have different personal preferences, but in your situation, I'd probably fit a 3.44, which is what I currently run in my own car that's also on 10" wheels. It's a good all-round compromise for acceleration and cruising, in my opinion. I've also previously had a 3.7, as well as a 3.1 geared Mini, and I think 3.44 is a nice "sweet spot" for a car which is used occasionally for different driving situations a bit like you mentioned. On a side note, the 3.1 diff was lovely in my old 998 which was used as a daily driver with plenty of motorway journeys and long trips, since it dropped the revs and noise right down, but you do sacrifice some acceleration for this of course.

3.44 diff…285 ish duration cam….1.5 carbs to look right..or 1.75 HIF for simplicity….or 40 / 45 DCOE Weber for ultimate performance if you want to cut the bulkhead and accept the induction noise.

Thank you both for the guidance.
Essentially it will/might a B road car, not many motorways where I live.
Im still weighing things up at the moment as I already have a nicely tuned 998.

I've got a 3.1 diff in my 1330 'S' (standard S 510 cam, standard 1 1/4 carbs, K & N's, Maniflow LCB and single box RC40).

Even with a higher diff it still accelerates very well and of course cuts the revs down whilst cruising. Sounds awesome too..I love it

Compared to my 998 Cooper, it's like comparing chalk and cheese.