The best way to do it is convert the metric part of the tyre size to imperial, so a 195/530 13 is 195mm wide 530mm tall on a 13'' wheel


converted to imperial


20.8'' tall 7.6'' wide 13'' wheel


Converted back in to UK tyre size is approx. 195/50/13


I use the below site to convert the imperial size back to UK sizing, you can also use the tyre size comparison to see how the new tyre looks against your original and the difference in speed.


https://tiresize.com/calculator/