{{ summaryHeading }}
{{ primaryDisplay }}
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Original New {{ stage.deltaLabel }}
Enter the tire size your speedometer, gearing, and clearance are currently based on.
Enter the tire size you want to buy or test against the original size.
Pick the speedometer reading you want converted to actual road speed.
{{ referenceSpeedDisplay }}
Use strict for OEM/AWD-sensitive swaps, daily for normal replacements, or lifted/off-road for deliberate size changes.
Choose a known comparison to reset the two sidewall size fields.
Adds drivetrain-specific review language without changing the tire geometry.
The width check flags when the new tire adds more than this much section width per side.
mm
Show both units, metric first, or imperial first in result tables and exports.
MeasurementOriginalNewChangeUseCopy
{{ row.metric }} {{ row.original }} {{ row.newValue }} {{ row.change }} {{ row.use }}
CheckSignalActionCopy
{{ row.check }} {{ row.signal }} {{ row.action }}
Speedometer readsActual speedDifferenceOdometer effectCopy
{{ row.reading }} {{ row.actual }} {{ row.difference }} {{ row.odometer }}
Chart unavailable
The tables and JSON still contain the full tire comparison.
{{ jsonOutput }}
Tags: Automotive
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Introduction

Tire size changes affect more than appearance. Overall diameter changes ride height, effective gearing, speedometer reading, odometer accumulation, anti-lock brake assumptions, and clearance around the fender, strut, control arm, and liner. Width changes can add grip and stance, but it can also reduce clearance or require a wheel width and offset that the vehicle does not have.

Metric tire markings such as 225/55R17 describe section width, sidewall aspect ratio, and rim diameter. Flotation markings such as 33x12.50R15 start from the advertised overall diameter and width. Both systems can be converted into the same geometry: sidewall height, rim diameter, overall diameter, circumference, and revolutions per distance.

The practical question is whether the replacement stays inside a reasonable change band for the vehicle. A small diameter change may be acceptable for many daily-driver fitments, while all-wheel-drive systems, tight wheel wells, brake clearance, load rating, and speed rating can make a seemingly close replacement unsuitable. A careful comparison treats geometry as the first screen, then checks vehicle-specific fitment and safety requirements.

Original and replacement tires compared by diameter and rolling circumference.
Diameter, width, and rolling circumference all move together when a tire size changes.

How to Use

Enter the original tire size from the vehicle placard, owner’s manual, or current approved fitment. Enter the replacement size using either metric format, such as 245/50R18, or flotation format, such as 33x12.50R15. The fields accept common tire-size punctuation and normalize the dimensions for comparison.

Set the displayed speed and speed unit to see how the tire change affects the actual road speed. Choose a review band that matches the use case: strict for conservative fitment review, daily for ordinary replacement comparisons, or off-road for larger diameter changes that still need mechanical verification.

Use the sensitivity and clearance controls when the vehicle has all-wheel drive, tight suspension clearance, aftermarket wheels, or known fender constraints. Review the size summary, fitment checks, speedometer table, and map together before using the JSON or copied notes in a parts list or quote request.

Interpreting Results

The diameter percentage is the headline fitment number. Positive diameter change means the replacement tire is taller; negative change means it is shorter. Actual speed rises when the replacement circumference is larger because the vehicle travels farther per wheel revolution than the speedometer expects.

Width difference and sidewall difference describe separate risks. Extra width can reduce inner or outer clearance even if the diameter is close. A shorter sidewall can sharpen steering response but reduce rim protection and ride compliance. A taller sidewall can soften impact harshness but may increase sidewall flex and clearance demand.

The review status is a geometry screen, not a final approval. Load index, speed rating, wheel width, wheel diameter, wheel offset, brake clearance, suspension travel, and manufacturer limits still need to match the vehicle.

Technical Details

Metric and flotation formats are converted into millimeters and inches so both systems can be compared on the same basis. Circumference is calculated from overall diameter, and speedometer correction is calculated from the ratio between new and original circumference.

Formula Core

For a metric tire with width w in millimeters, aspect ratio a, and rim diameter r in inches:

Sidewall=w×a100 Diameter=(r×25.4)+2×Sidewall

Rolling circumference and road-speed correction are:

C=π×Diameter Actual speed=Displayed speed×CnewCoriginal

Revolutions per mile use the circumference in inches:

Revs per mile=63360Cinches
Measurement Fitment meaning
Diameter Affects ride height, gearing, speedometer, and wheel-well clearance.
Width Affects inner clearance, outer poke, wheel compatibility, and steering load.
Sidewall Affects ride feel, impact protection, and response.
Circumference Affects actual speed and odometer accumulation.

Accuracy Notes

Published tire dimensions are nominal. Actual mounted diameter and section width can vary by tire model, rim width, inflation pressure, load, tread depth, and manufacturing tolerance. New tread can also make a tire taller than a worn tire with the same size marking.

The comparison does not replace test fitting or manufacturer guidance. All-wheel-drive systems may require closely matched rolling circumference, and some vehicles have narrow tolerance for tire diameter or tread-depth differences across axles. Always verify load index, speed rating, wheel width range, rim diameter, and clearance under steering and suspension compression.

Worked Example

A change from 225/55R17 to 245/50R18 increases section width and changes sidewall height while also increasing rim diameter. The calculator converts both sizes to total diameter and circumference, compares the percentage change, and applies that ratio to a displayed speed. If the replacement circumference is larger, a speedometer showing 60 mph will correspond to a higher actual road speed.

Common Questions

Is a small diameter difference always acceptable?

No. It may be geometrically close, but vehicle drivetrain, load rating, clearance, wheel fitment, and manufacturer rules still control suitability.

Why can the speedometer change if the rim is bigger?

The speedometer follows wheel revolutions. A tire with a larger rolling circumference travels farther per revolution, so actual speed rises for the same indicated speed.

Can flotation and metric tire sizes be compared directly?

Yes after conversion. Both markings can be reduced to width, rim diameter, overall diameter, sidewall height, and circumference.

Glossary

  • Aspect ratio: Sidewall height expressed as a percentage of section width.
  • Section width: Tire width measured sidewall to sidewall under specified mounting conditions.
  • Rolling circumference: Distance the tire travels in one revolution.
  • Review band: The selected percentage range used to flag diameter changes for closer review.