I want to use a wide tire for my car. According to my readings, tires could be used with a 20mm +/- variation than the stock ones.
So tire width (internal or outer) needs to match rim width? Also tire internal diameter is a matter?
Tires are designed to work properly with a certain size (i.e. width) rim. Too narrow a rim leads to the tire being "squished" and the sidewalls compressed. To wide a rim leads to the tire being "stretched". Either condition leads to less than optimal tire performance and wear and MAY BE DANGEROUS.
You should follow the tire manufacturer's recommendation for the proper rim sizes for a given size of their tire.