175/70 is about the recommended limit on stock rims, but your local tyre bloke would know. Standard HR rims will take 185/80, which are a nice big tyre, not that much smaller than original "conventional" tyres. 175/70 have a
significantly smaller rolling circumference, which will cause the speedo to over-report driving speed (and the motor to work harder to reach the same "real" speed).
What's wrong with 4" crossply tyres, anyway? Much more exciting
cheers
RET
[ Get caught up for 5 minutes doing something else and look how many replies appear whilst you're still composing. :-/ ]