Tire Size Calculator & Comparison
Compare two tire sizes to see how diameter, width, circumference, speedometer reading, and ground clearance change. Enter your current and new tire sizes below.
Enter two tire sizes above to compare diameter, speedometer error, and clearance changes.
Understanding Tire Sizes
Tire sizes follow a standardized format that encodes three critical measurements. In a size like 265/70R17, the first number (265) is the tire width in millimeters measured from sidewall to sidewall. The second number (70) is the aspect ratio — the height of the sidewall expressed as a percentage of the width. So a 265/70 tire has a sidewall height of 265 × 0.70 = 185.5mm. The letter R indicates radial construction (virtually all modern tires), and the final number (17) is the rim diameter in inches. From these three numbers, you can calculate the overall diameter, circumference, revolutions per mile, and ground clearance change when switching sizes.
When you change tire sizes, the most important consideration is the overall diameter difference. A tire with a larger diameter covers more ground per revolution, which means your speedometer will read slower than your actual speed. Conversely, a smaller diameter tire makes your speedometer read faster than actual speed. Most manufacturers and tire shops recommend staying within 3% of the original diameter to avoid significant speedometer error, potential ABS and traction control issues, and warranty concerns. Beyond 3%, you may need a speedometer recalibration.
Ground clearance changes by exactly half the diameter difference. If your new tire is 1 inch larger in overall diameter, your vehicle sits 0.5 inches higher. This matters for off-road capability, but also for fitting the larger tire within your wheel wells. Width changes affect clearance too — a wider tire may rub on suspension components, fender liners, or the wheel well itself, especially at full steering lock or when the suspension compresses over bumps. Always check clearance with the wheels turned fully in both directions before driving on new tires.
Speedometer Correction
Your vehicle's speedometer calculates speed based on how fast the transmission output shaft rotates, using the original tire size to convert rotations to miles per hour. When you install tires with a different circumference, the speedometer reading becomes inaccurate. The formula is straightforward: actual speed = displayed speed × (new tire diameter ÷ original tire diameter). For example, if your new tires are 3% larger in diameter, when your speedometer reads 60 mph, you are actually traveling about 61.8 mph. This affects not only your speedometer but also your odometer — you will accumulate fewer miles than actually driven, which can affect resale value, warranty mileage, and maintenance intervals.
Modern vehicles with electronic speedometers can often be recalibrated by a dealer or with an aftermarket programmer. Older vehicles with cable-driven speedometers may need a speedometer gear change. Some states require accurate speedometers for vehicle inspection, so check your state's regulations before making significant tire size changes.
Frequently Asked Questions
How much tire size difference is acceptable?
Most manufacturers recommend staying within 3% of the original tire diameter. Within this range, speedometer error is minimal and you're unlikely to have clearance issues, ABS problems, or warranty concerns. Beyond 3%, you should consider speedometer recalibration and check for clearance.
Will bigger tires affect my gas mileage?
Yes, larger and especially wider tires typically reduce fuel economy by 1-3% due to increased rolling resistance and weight. The effect varies by tire construction, tread pattern, and how much larger the new tires are. Highway fuel economy is more affected than city driving.
Can I put different size tires on my car without any modifications?
It depends on how much the size changes. Within 3% diameter and similar width, most vehicles can accommodate the change without modifications. Larger changes may require wheel spacers, fender trimming, lift kits, or leveling kits. Always check clearance at full steering lock and full suspension compression.