Tire Size Calculator
Decode any tire size code into real-world dimensions, and compare two sizes for speedometer error.
🛞 What is the Tire Size Calculator?
The tire size calculator turns the cryptic code printed on a tire's sidewall, like 225/45R17, into real-world numbers: sidewall height, overall diameter, circumference, and how many times the tire rotates per mile. Enter the three numbers from the tire code and it does the geometry for you.
Drivers reach for this tool in a few common situations. Someone shopping for replacement tires wants to know if a different size will still fit their fender well and keep their odometer accurate. An off-road enthusiast planning a lift kit wants to see exactly how much taller a new tire will sit. A mechanic double-checking a customer's plus-size fitment wants the overall diameter without pulling out a tape measure. All of these come down to the same three-number decode.
The calculator also includes a Compare mode. Enter an original and a new tire size, plus what your speedometer currently reads, and it shows the diameter change and the resulting speedometer error, so you know whether a size swap will make your speedometer read meaningfully high or low.
This tool is distinct from a full speedometer recalibration calculator. It focuses on decoding the tire size notation itself into physical dimensions, with the speedometer check as a convenient extra. If you have also changed your axle gear ratio, or you already know your tire diameters and just need the speedometer math, the dedicated Speedometer Gear Calculator on this site covers that combined case.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 - Decode a common sedan tire, 225/45R17
Example 2 - Decode a plus-size fitment, 235/40R18
Example 3 - Compare 225/45R17 to 235/40R18 at an indicated 60 mph
Example 4 - Compare an off-road upsize, 265/70R17 to 275/65R18 at an indicated 65 mph
❓ Frequently Asked Questions
🔗 Related Calculators
What do the numbers in a tire size mean?
In a code like 225/45R17, 225 is the tire width in millimetres, 45 is the aspect ratio (sidewall height as a percent of width), R means radial construction, and 17 is the rim diameter in inches. This calculator decodes those three numbers into sidewall height, overall diameter, and circumference.
How do I calculate tire sidewall height?
Sidewall height (mm) = width x aspect ratio / 100. For a 225/45R17 tire, sidewall = 225 x 45 / 100 = 101.25 mm. This is the height of the tire's side from the rim to the tread, not the overall radius.
How do I calculate overall tire diameter?
Overall diameter (in) = rim diameter + 2 x sidewall height / 25.4. For a 225/45R17: 17 + 2 x 101.25 / 25.4 = 17 + 7.97 = 24.97 inches. The rim contributes its own diameter, and the sidewall is added twice, once on each side of the wheel.
What is a plus-size tire fitment?
A plus-size fitment swaps to a larger rim with a lower-profile tire that keeps the overall diameter close to the original, such as moving from 225/45R17 to 235/40R18. This calculator's Compare mode checks whether a plus-size swap keeps the speedometer accurate.
How much does changing tire size affect my speedometer?
The speedometer error percent equals (new diameter divided by old diameter minus 1) times 100. Going from 225/45R17 (24.97 in) to 235/40R18 (25.40 in) changes the diameter by about 1.72 percent, so an indicated 60 mph is really about 61.0 mph.
Does tire width alone change my speedometer reading?
Not directly. Speedometer accuracy depends only on overall diameter, not width. Two tires of very different widths can have the same diameter if the aspect ratio is adjusted, in which case the speedometer stays accurate.
How many times does a tire rotate per mile?
Divide 63,360 inches (the number of inches in a mile) by the tire's circumference in inches. A 225/45R17 tire has a circumference of about 78.45 inches, so it rotates roughly 807.6 times per mile. Smaller tires rotate more often; larger tires rotate less often.
Is this calculator the same as the Speedometer Gear Calculator?
No. This Tire Size Calculator decodes the printed tire code (like 225/45R17) into sidewall, diameter, circumference, and revs per mile, then offers a quick tire-only speedometer check. The Speedometer Gear Calculator takes diameters you already know and also factors in an axle gear ratio change, which this tool does not cover.
Why does my speedometer read low after fitting bigger tires?
A larger-diameter tire covers more ground per revolution. Since the speedometer counts wheel or driveshaft rotations and assumes the original tire's diameter, it under-counts your true distance and shows a lower speed than you are actually travelling.
How do I find the overall diameter of my current tires without the code?
Measure from the ground to the center of the wheel hub and double it, or look up the size printed on the sidewall (like 225/45R17) and compute it precisely: diameter equals rim diameter plus twice the sidewall height, where sidewall height is width times aspect ratio divided by 100, converted from millimeters to inches.
Can I compare tires measured in different units?
Enter both tires' width in millimetres, aspect ratio as a percent, and rim diameter in inches, which is the standard way tire sizes are printed worldwide. The calculator handles the millimetre-to-inch conversion internally.