miata tyre calculator
the Miata tyre calculator is a really good tool to use

remember
155 is the tyre width in mm
80 is the percentage of the width. this is the sidewall height
R12 is the rim size in inches.

a 155/80/R12 has a tyre diameter of 548mm (about)
a 165/75/R12 has a tyre diameter of 547.5mm (about)

The rolling dimeter is identical for intents and purposes so either tyre is fine
the 165/75/12 is slightly wider but not enough to make ahuge difference.
All things bieng equal, a taller tyre will get more traction in a straight line then a shorter one. (longer footprint) Think drag tyres as an extreme example)

It will affect it, but not more than the factory tolerance.

165/70R12 -- 70 series -- is available in USA, more than one brand
See Tires - where to buy 12" tires

I have 165/55R12, they are now out of production, but you can still buy four new ones.

ok thanks would they be similar size will it affect the speedo readings?

thanks!

If you go to 155/80/R12 tyres you can still get them without too much hassle. Not mega common, but still available.

I can get them in Perth at least.

they shouldnt be that hard.

hi i have a datto 120y with original 12 inch wheels and i need new tyres for roadworthy but i cant find these size tyres anywere 165/75/r12 i asked nealy every one they say they never seen this size all just sound confused and on my old tyre thats the size it says on it!

does this mean there out of production?

anyone got 4stud rims with good tyres here in melbourne?

thanks!