Tire Size Calculator
Calculate tire diameters, section widths, sidewall profiles, revolutions, speedometer offset, and fitment clearances with synchronized real-time scale graphics.
When dashboard indicates the standard benchmark speed, your actual road velocity with Tire 2:
| Parameter | Tire 1 (Stock) | Tire 2 (New) | Difference (Δ) |
|---|---|---|---|
| Overall Diameter | 24.88 in | 24.99 in | +0.11 in (+0.4%) |
| Section Width | 8.07 in | 8.86 in | +0.79 in (+9.8%) |
| Sidewall Height | 4.44 in | 3.99 in | -0.45 in (-10.1%) |
| Circumference | 78.17 in | 78.50 in | +0.33 in (+0.4%) |
| Revolutions / Mile / km | 811 /mi | 807 /mi | -4 /mi (-0.4%) |
| Wheel Rim Size | 16.0 in | 17.0 in | +1.0 in |
| Rim Width Range | 5.5 - 7.5 in | 7.0 - 8.5 in | Ideal: 6.5" vs 7.5" |
Tire Sizing & Fitment Engineering Principles
Choosing the right tire or wheel combination requires maintaining an overall rolling diameter within ±3.0% of standard OEM dimensions to preserve transmission shift points, ABS electronic braking logic, and accurate odometer calibration.
Section Width (mm)
The cross-section width of an inflated tire measured in millimeters from sidewall to sidewall. Wider tires create a broader contact patch for superior lateral grip, braking, and launch traction.
Aspect Ratio (%)
The ratio of sidewall height to section width. A "45" profile means sidewall height is 45% of width. Lower profiles increase steering precision and cornering rigidity at the expense of ride cushioning.
Plus-Sizing Rule
When stepping up in rim diameter (e.g. from 16" to 17" or 18"), tire aspect ratio must decrease proportionally so overall rolling diameter remains identical, safeguarding gearbox ratios and speedometer accuracy.
How CalculatorTireSize.com Works
Tire dimensions come from the sidewall code. Sidewall height = width × aspect ratio / 100. Overall diameter = rim diameter + 2 × sidewall height, with all lengths in the same unit.
Enter your current tire
Select the width, aspect ratio and rim diameter of the stock tire (Tire 1), exactly as printed on the sidewall, for example 205/55R16.
Choose the new size
Set the size you are considering as Tire 2, or tap a quick preset for common plus-sizing, staggered and truck-lift upgrades.
Compare every dimension
The matrix shows diameter, width, sidewall, circumference, revolutions per mile or km and rim width range, in inches or millimetres.
Check speedometer and fitment
See your true speed at common indicated speeds, the change in ride height and fender gap, and whether the new size stays inside the ±3% safety window.
Tire Diameter Calculator
Find how tall a tire stands. Read it in inches or mm.
Tire Revolutions per Mile Calculator
See how many times a tire turns in one mile.
Metric Tire Size Decoder
Read a code like 225/45R17. Each part is explained.
Tire Size Comparison Calculator
Put two tire sizes side by side. See every change.
Equivalent Tire Diameter Finder
Find sizes that are about as tall as yours.
Speedometer Error Calculator
See how a new size changes what the speedometer shows.
Winter Wheel Package Calculator
Plan a set of winter wheels and tires.
Equivalent Axle Ratio Calculator
Find the axle ratio that brings back stock gearing.
Tire Service Description Comparator
Compare the load and speed code of two tires.
Motorcycle Tire Size Calculator
Read a motorcycle tire code.
Tread Depth Unit Converter
Switch tread depth between mm and 32nds.
Looking for a complete categorized directory?
Filter, search, and view formulas for all 136 specialized tire and wheel tools.
Tire Size Calculator FAQ
Answers to common questions about reading tire sizes, plus-sizing, speedometer error and fitment.
How do I read a tire size like 225/45R17?
225 is the section width in millimetres, 45 is the aspect ratio (the sidewall height is 45% of the width), R means radial construction, and 17 is the wheel (rim) diameter in inches.
How is overall tire diameter calculated?
Sidewall height = width × aspect ratio ÷ 100. Overall diameter = rim diameter + 2 × sidewall height. For 225/45R17: 225 × 0.45 = 101.25 mm (3.99 in) per sidewall, so the diameter is 17 + 2 × 3.99 ≈ 24.97 in.
How much can the tire diameter change safely?
A common guideline is to keep the new overall diameter within ±3% of the original. Larger changes can upset the speedometer and odometer, ABS and traction-control readings, automatic-gearbox shift points and clearance in the wheel arch.
How does a bigger tire affect my speedometer?
A larger tire covers more ground per revolution, so the speedometer reads low. Actual speed ≈ indicated speed × new diameter ÷ old diameter. With a tire 3% larger, an indicated 60 mph is really about 61.8 mph.
What is plus sizing?
Plus sizing means fitting a larger-diameter wheel and a lower-profile tire so the overall diameter stays about the same. For example, going from 205/55R16 to 225/45R17 (+1) changes the overall diameter by less than 0.5%.
Which rim width fits my tire?
Every tire size has an approved rim width range set by the tire manufacturer. As a rule of thumb the ideal rim is about 80–85% of the tire's section width; for a 225 mm tire the approved range is typically about 7.0 to 8.5 inches. The calculator shows the range for both tires.
Is the tire size calculator free, and is my data stored?
Yes, it is completely free with no sign-up. All calculations and graphics run in your browser, so the sizes you enter are never sent to or stored on a server.