For Alto, you can easily opt for 165/70/R12. I am using 165/65/R13 on my Alto without any issue so far! Drove 68K KM on this particular size and Allhamdoulillah never encountered any issues. However, when it comes to economy, there is a slight decrease in it as compared to OEM Size (145/70/R12). With OEM if you get 170km/cylinder, then with 165/65/R13, u'd get 150km/cylinder.
Coming to Mehran, I had 165/65/R13 on my Mehran which i drove for 40K KM, again never encountered any issues with it. However, there was a decrease in economy, same as Alto's.
I drove both the cars without any passengers in the back seat, therefore i never had any issues. But if Alto or Mehran need to be used as family cars, stick to 165/70/R12 preferably with alloy rims. I am suggesting alloy rims, because there are relatively light as compared to steel OEM rims. Having 165/70/R12 with alloy wheels will have the same effect as 145/70/R12 with steel rims since the weight of the wheel will remain same. With 165/70/R12 you'd have a significant increase in grip and stability of the car on highways and negligible effect of the consumption.
these are just my two cents which are based on thorough experience.
Regards.