Understanding Tire Size Numbers
A tire size such as 225/60R16 contains three important measurements. The first number, 225, is the nominal section width of the tire in millimeters. The second number, 60, is the aspect ratio, meaning the sidewall height is approximately 60% of the tire's section width. The final number, 16, is the wheel diameter in inches.
Because the sidewall is based on a percentage of the tire's width, changing either the width or aspect ratio can change the tire's overall diameter.
What Happens When Tire Diameter Changes?
A larger-diameter tire travels farther with each revolution. This means it makes fewer revolutions over a given distance and can cause the vehicle's actual road speed to be higher than the speedometer indicates if the speedometer was calibrated for the original tire.
A smaller-diameter tire does the opposite. It travels a shorter distance per revolution, makes more revolutions over a given distance, and can cause actual road speed to be lower than the indicated speed.
Changing tire diameter can also affect the odometer because the vehicle may calculate distance based partly on wheel or drivetrain rotation.
Tire Size and Ground Clearance
Changing overall tire diameter can theoretically change vehicle ground clearance by approximately half of the diameter difference. For example, increasing overall tire diameter by one inch can raise the axle approximately half an inch.
This is only a theoretical tire-radius change. Actual vehicle clearance and fitment depend on many other factors, including suspension design, wheel dimensions, tire construction, vehicle load, and available space around the tire.
Comparing Replacement Tire Sizes
When considering a different tire size, overall diameter is often one of the most useful numbers to compare. Two tires with different widths, aspect ratios, and wheel diameters can still have similar overall diameters.
The calculator above lets you compare those dimensions directly and estimate how the change could affect tire revolutions, ride height, speedometer readings, and odometer distance. Actual tire dimensions can vary by manufacturer and tire model, so manufacturer specifications should be checked before choosing a replacement tire.