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Calculate speedometer error and ride height changes after upgrading your tires.
Calculation Formulas
Sidewall Height: H = W × R% / 100
Overall Diameter: D = Rim × 25.4 + 2 × H
Circumference: C = π × D
Revolutions per Kilometer: N = 1,000,000 / C
Please enter positive real numbers. Aspect ratio typically ranges from 25 to 90, and rim diameter is in inches.
Overview
Understand what the tool solves, how it works, and the boundaries of its data.
Enter the original and comparison tire as three values: section width in millimeters, aspect ratio as a percent, and rim diameter in inches. In a size such as 205/55R16, 205 is the nominal width, 55 means the sidewall height is 55% of that width, and 16 is the rim diameter. These are nominal dimensions used for a geometric estimate; ISO 4000-1:2024 covers designation, dimensions, and load ratings for metric-series passenger-car tires.
The calculator reports estimated sidewall height, overall diameter, circumference, and revolutions per kilometer for each input, then shows comparison differences. Its calculation is:
Sidewall height (mm) = width × aspect ratio ÷ 100
Diameter (mm) = rim diameter (in) × 25.4 + 2 × sidewall height
Circumference (mm) = π × diameter
Revolutions/km = 1,000,000 ÷ circumference (mm)
Diameter change (%) = (comparison diameter − original diameter) ÷ original diameter × 100
For example, 205/55R16 gives an estimated diameter of about 631.9 mm; 225/45R17 gives about 634.3 mm, a difference of roughly 2.4 mm or 0.38%. If the dashboard reference is 60 mph, the comparison speed estimate scales by the same diameter ratio: about 60.23 mph. The speed field accepts a numeric value without unit conversion, so interpret its result in the same speed unit you entered.
Guide
Follow the workflow and verify inputs and outputs with practical examples.
Enter width in mm, aspect ratio as a number such as 55, and rim diameter in inches. The preview combines them into a size string for quick checking.
Enter the candidate tire's width, aspect ratio, and rim diameter in the second group. You can compare its nominal geometry with the original values.
Enter the speed shown on your dashboard, using mph or km/h consistently with how you want to interpret the estimate. Choose how many decimal places are displayed for dimensional results.
Read the diameter percent and absolute difference alongside sidewall, circumference, and revolutions-per-kilometer changes. The comparison speed estimate is calculated from the original displayed speed and the diameter ratio.
Use cases
See how the tool fits into real work and everyday tasks.
Before asking a tire shop about a replacement, compare its nominal dimensions with the current size. Share the vehicle's exact make, model, year, and trim with the shop for a separate fitment check.
Set a familiar dashboard reading, such as 60 mph, and see how the candidate's nominal diameter changes the proportional estimate. This helps frame a question about speed indication; it does not measure the vehicle.
Q&A
Find concise answers to common questions and confusing cases.
No. The calculator compares nominal dimensions only. It does not check wheel width, clearance, load rating, manufacturer approval, or vehicle-specific compatibility.
At a fixed tire rotation rate, a larger nominal diameter covers more distance per revolution. The page estimates speed by multiplying the dashboard reference by comparison diameter divided by original diameter.
The fields are separate: width, aspect ratio, and rim diameter. For 205/55R16, enter 205, 55, and 16 respectively.
Notes
Review scope, result limitations, and important precautions before use.
Actual tire dimensions can differ from nominal estimates by model, construction, load, inflation, and wear. The displayed ±3% threshold is a visual warning threshold in this calculator, not a universal legal limit or proof of vehicle compatibility. Confirm the approved size, load capacity, wheel fit, clearances, and local requirements with the vehicle manufacturer or a qualified tire professional before changing tires. Do not treat the speed estimate as a calibrated speedometer measurement.
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