If I get an 18 tooth pinion with the 64 tooth crown wheel I'd get 3.55, or should I just bite the bullet and get the correct combination for 3.44?

No can do.

You can't mix and match pinions and gears. The tooth profiles are not machined to match each other and they will fail.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

18T pinion won't go with your 64T crownwheel.
18T is only used with a 62T crownwheel, and be careful, because there are 3 of these.
The one you need is marked 22A411

@Mick, you can use the other pinions with various crownwheels, but NOT the 18T one.
eg the same 17T, 22A399 pinion is used with a 64T crownwheel (3.765) and a 62T (3.647).

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Cheers Doc.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

A good second hand 18/62 crown wheel and pinion should not cost you to Much
I like the 3.1:1 Myself but they are around $300 for a new set, Others may disagree

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" A Hundred Years From Now It will not Matter what my Bank Acount Was,
The Sort Of House I Lived in, Or The Kind Of Car I drove ... But The World May Be A Better Place
Because I was Once Important In The Life Of A Child"

$300 isn't too bad. So John sell me the 3.1:1, what's the good, bad and ugly of this set up?

3.1 is a good ratio in a 1275, particularly if you do much highway work.

Me? I run a 3.647, bwahahaha....

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

I Like them because they are good for cruising, I find I am not thinking I need to change
gears but find I am already in top gear, and the motor is not revving itself to death @ higher speeds
the down side is You lose a little of the mark.
others May have a different opinion.

_________________
" A Hundred Years From Now It will not Matter what my Bank Acount Was,
The Sort Of House I Lived in, Or The Kind Of Car I drove ... But The World May Be A Better Place
Because I was Once Important In The Life Of A Child"