The "R" in the tire size designation stands for "radial", and the number designates the nominal diameter of the tire in inches. The aspect ratio is the percentage of the tire's width that is represented by the height of the sidewall. For example, a tire with a size designation of "R15 225/55" has a nominal diameter of 15 inches and an aspect ratio of 55%.
If you were to replace R15 tires with R14 tires, the aspect ratio of the tire would change. This would cause the speedometer to read incorrectly, as it would be calibrated for the original tire size. The vehicle could also become unstable, as the wheel and tire combination would be outside of the manufacturer's specifications.
For safety reasons, it is important to only use the tire size that is specified by the vehicle manufacturer.
