235/60's seem to be the most popular and most frequently seen replacement size - that's what my '69Z had on it when I bought it 12 years ago; it has the original springs, and I had occasional rubs at 12 o'clock at the top of both front wheel opening moldings at full lock (like entering a driveway or parking space). I replaced them with 215/65's, which are dimensionally almost identical to the original E70-15's, and no longer have any rubbing (and the speedo is dead-on accurate). If you have new springs and your car sits a bit higher than mine, 235/60's may not be a problem.