This is a tire size calculator for the same or different rim sizes. It allows you to compare different tire and rim sizes and tells you the expected speedometer difference at any speed you pick. You can see the difference in diameter (in mm) and the sidewall height (in inches). Of course, there is no guarantee that the tires you’re looking at will fit your car or rims – you’ll have to check that out yourself. See the wheel / rim offset calculator to determine the best offset for your new rims.
When buying rims it is always advised to test mount the rims on the vehicle before mounting tires to ensure you have sufficient clearance to the fender and shocks/struts.