Probably the best tyre calculator i found online yet
here
In the upper box you enter the factory specifications of what the vehicle was designed to run on, tyres and wheels - vehicle stock tyre size
In the lower 4 boxes (4 attempts), you can enter what you are thinking of putting on tyres and alloys.
It will calculate all the width offset stuff but... it will tell you lots more, overall diameter change , speedo inaccuracy...etc etc
here
In the upper box you enter the factory specifications of what the vehicle was designed to run on, tyres and wheels - vehicle stock tyre size
In the lower 4 boxes (4 attempts), you can enter what you are thinking of putting on tyres and alloys.
It will calculate all the width offset stuff but... it will tell you lots more, overall diameter change , speedo inaccuracy...etc etc