Compare two tire sizes and check speedometer accuracy, diameter difference, and fitment.
A tire marked 225/65R17 contains three key measurements. The first number (225) is the tread width in millimeters. The second number (65) is the aspect ratio, meaning the sidewall height is 65% of the tread width. The letter R indicates radial construction (virtually all modern tires). The last number (17) is the wheel diameter in inches. To calculate the overall tire diameter, you need to convert the sidewall height to inches, double it (for both top and bottom sidewalls), and add the wheel diameter.
Changing tire size affects several vehicle systems. Your speedometer is calibrated to your original tire size, so a larger tire makes the speedometer read slower than your actual speed, while a smaller tire makes it read faster. Your odometer is similarly affected, meaning your recorded mileage will be slightly off. ABS and traction control systems use wheel speed sensors that assume a specific tire size, and significant changes can cause these systems to behave unpredictably. Most manufacturers recommend staying within 3% of the original tire diameter to keep all systems working properly.
Plus sizing means going up one or two wheel sizes while decreasing the tire profile to maintain approximately the same overall diameter. For example, moving from 225/65R17 to 235/55R18 keeps the diameter nearly identical while giving a wider contact patch and a sportier look. The tradeoff is a firmer ride (less sidewall cushion), increased risk of wheel damage from potholes, and typically higher cost for both tires and wheels. Always verify that the new combination maintains a diameter within 3% of the original.
Truck and SUV upsizing: Owners often move from stock sizes like 265/70R17 to larger options like 285/70R17 or 275/65R18 for improved off-road capability and appearance. These changes increase diameter by 1-4% and may require a leveling kit or lift for clearance. Sedan plus-sizing: Moving from 205/55R16 to 225/45R17 is a common upgrade that maintains similar diameter while adding grip and style. Winter tire downsizing: Some drivers go down a wheel size for winter (e.g., from 18" to 17") to get a taller sidewall that absorbs bumps better and fits narrower snow tires. The Fuel Cost Calculator helps estimate the fuel economy impact of tire changes, since wider tires typically increase rolling resistance.
When your speedometer reads 60 mph with tires 3% larger than stock, your actual speed is about 61.8 mph. Over time, this adds up on your odometer too, meaning your car has actually traveled farther than the odometer shows. For truck and SUV owners with significantly oversized tires (33" or 35" on a vehicle that came with 29" tires), the speedometer error can exceed 10%, which is a meaningful safety and legal concern. Many modern vehicles allow speedometer recalibration through a dealer or aftermarket tuner. The Unit Converter handles other measurement conversions.