How to read a tire size
In 225/45R17, 225 is the nominal section width in millimeters. The 45 is the aspect ratio, meaning the sidewall height is 45% of the section width. The 17 is the wheel diameter in inches.
Calculating overall diameter
First calculate sidewall height in millimeters, divide by 25.4 to convert it to inches, double it for the top and bottom sidewalls, then add the wheel diameter.
Why changing diameter affects the speedometer
The speedometer expects a certain distance traveled per wheel revolution. A taller tire covers more ground per revolution, so actual speed is higher than indicated. A shorter tire covers less ground, so actual speed is lower.
Fitment is more than diameter
Section width, wheel width, offset, suspension shape, alignment and body clearance all matter. Two tires with nearly identical diameter can fit very differently if their widths are different.