Tire Size Comparison Calculator
Enter two tire sizes (width/aspect ratio/rim diameter) to compare their overall diameter and the resulting speedometer difference.
Last updated: September 5, 2026
Frequently asked questions
How do I read a tire size like 225/45R17?
225 is the tire width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the rim diameter in inches.
Why does changing tire size affect my speedometer accuracy?
Your speedometer is calibrated based on your original tire's diameter and how many times it rotates per mile. A larger-diameter tire covers more distance per rotation, so your speedometer (still counting rotations the same way) will read lower than your actual speed - and a smaller tire will make it read higher.
How much difference in tire diameter is considered acceptable?
Many tire professionals recommend staying within about 3% of the original diameter to avoid significantly affecting speedometer accuracy, ABS/traction control calibration, and ground clearance.
Does tire width alone tell you the overall size?
No - width is just one factor. Overall diameter depends on width, aspect ratio (sidewall height), and rim diameter together, which is why two tires with different width/aspect/rim combinations can end up nearly the same overall size.
How much bigger is a 235/40R18 tire than a 225/45R17?
It depends on the specific combination - you need to calculate overall diameter for both (width × aspect ratio × 2, converted to inches, plus rim diameter) since a wider tire with a lower aspect ratio can end up close in overall size to a narrower tire with a taller sidewall, which is exactly the comparison this calculator performs.
If my new tire's diameter is 2% larger, will my speedometer read low or high?
Your speedometer will read lower than your actual speed - a larger tire covers more ground per rotation, so at any given speedometer reading (based on wheel rotations) you're actually traveling faster than the display shows.
Is 'plus sizing' the same thing as what this calculator compares?
Yes - plus sizing refers to fitting a larger-diameter wheel with a lower-profile (lower aspect ratio) tire while keeping the overall tire diameter close to stock, which is exactly the width/aspect/rim tradeoff this calculator lets you check between two tire specs.