Tire Size Comparison Calculator

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Check the tire sizes

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Original tire size:

Enter the tire size your speedometer, gearing, and clearance are currently based on.

New tire size:

Enter the tire size you want to buy or test against the original size.

Compare speed at:

Pick the speedometer reading you want converted to actual road speed.

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Review band:

Use strict for OEM/AWD-sensitive swaps, daily for normal replacements, or lifted/off-road for deliberate size changes.

Sample comparison:

Choose a known comparison to reset the two sidewall size fields.

Vehicle sensitivity:

Adds drivetrain-specific review language without changing the tire geometry.

Per-side clearance cushion:

The width check flags when the new tire adds more than this much section width per side.

mm

Geometry display:

Show both units, metric first, or imperial first in result tables and exports.

Measurement	Original	New	Change	Use	Copy
{{ row.metric }}	{{ row.original }}	{{ row.newValue }}	{{ row.change }}	{{ row.use }}

Check	Signal	Action	Copy
{{ row.check }}	{{ row.signal }}	{{ row.action }}

Speedometer reads	Actual speed	Difference	Odometer effect	Copy
{{ row.reading }}	{{ row.actual }}	{{ row.difference }}	{{ row.odometer }}

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The tables and JSON still contain the full tire comparison.

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A tire size change alters the shape of the vehicle's contact with the road before it changes the look of the wheel well. Overall diameter changes rolling circumference, ride height, effective gearing, speedometer reading, odometer accumulation, and clearance at the fender liner, strut, control arm, and bumper edge. Section width changes side-to-side space and can create inner rub or outer poke even when the diameter looks close.

Most passenger and light-truck sidewall codes combine metric and inch units. In a size such as 225/55R17, 225 is section width in millimeters, 55 is sidewall height as a percentage of that width, and 17 is wheel diameter in inches. A flotation size such as 33x12.50R15 starts from the advertised overall diameter and section width in inches. Both systems can be reduced to the same geometry: sidewall height, wheel diameter, overall diameter, rolling circumference, and revolutions per distance.

Speedometer error comes from circumference, not from wheel diameter alone. A taller tire travels farther per wheel revolution, so a vehicle calibrated for the original tire will move faster than the speedometer indicates. A shorter tire does the opposite. Odometer readings shift in the inverse direction because the vehicle records revolutions, then assumes the original circumference.

Tire size change effects
Change	Main effect	Common oversight
Diameter	Ride height, gearing, speedometer, odometer, and wheel-well clearance.	Checking only the rim size instead of the full tire diameter.
Width	Inner clearance, outer fender position, steering sweep, and wheel compatibility.	Ignoring the per-side width change created by the new section width.
Sidewall	Impact cushion, steering response, rim protection, and ride feel.	Assuming plus sizing changes only appearance.
Rolling circumference	Actual speed, displayed distance, revolutions per mile, and drivetrain matching.	Treating a close visual fit as proof that speed and AWD tolerances are acceptable.

Review bands are only screening bands. A daily-driver swap that stays near the original diameter can still fail because of load index, speed rating, wheel width range, offset, brake clearance, tire-chain clearance, suspension travel, or manufacturer rules. All-wheel-drive and four-wheel-drive systems can be especially sensitive to mismatched rolling circumference across corners.

Original and replacement tires compared by diameter and rolling circumference. — Diameter, width, and rolling circumference change together, but each one affects a different part of fitment.

Nominal size math is a first pass. The real tire model, rim width, inflation pressure, vehicle load, tread depth, and manufacturing tolerance can make the mounted tire differ from the calculated dimensions.

How to Use This Tool:

Use the original size as the baseline the vehicle is already calibrated around, then compare the proposed size against that baseline.

Enter Original tire size from the vehicle placard, owner's manual, or a known approved current fitment. Metric codes such as 225/55R17 and flotation codes such as 33x12.50R15 are accepted.
Enter New tire size for the tire you want to test. If the summary says Check the tire sizes, correct the format or range before trusting any result.
Set Compare speed at and choose mph or km/h. The Speedometer Table will show actual road speed and the difference at common readings.
Choose a Review band. Use Strict OEM for close replacement checks, Daily driver for normal street swaps, and Custom review for deliberate truck, lift, or off-road changes.
Use Sample comparison when you want a known plus-size, winter, truck, or flotation scenario, then edit any field as needed.
Set Vehicle sensitivity. The all-wheel-drive setting caps the effective diameter band at 2% and adds drivetrain-specific caution text.
Set Per-side clearance cushion and Geometry display. The cushion is compared against half the section-width change, while the display setting controls whether result tables show metric, imperial, or both.
Read Size Summary first, then Fitment Checks, Speedometer Table, Fitment Zone Map, and JSON if you need a structured record.

Interpreting Results:

The diameter percentage is the main speedometer and ride-height clue. A positive value means the new tire is taller. A negative value means it is shorter. The ride-height change is half the diameter change because only the radius lifts or lowers the vehicle.

Within review band means the diameter change is inside the selected tolerance. It does not approve wheel width, offset, load rating, speed rating, brake clearance, or manufacturer fitment rules.
Fitment review means the diameter change is outside the selected band but no more than 5%. Check speedometer, gearing, clearance, and drivetrain requirements before buying.
Large size change means the diameter change is greater than 5%. Expect a stronger review for speedometer correction, gearing, clearance, spare fit, and suspension travel.
Width and clearance compares the per-side section-width change with the selected cushion. Wheel offset and actual tire shape can still move the tire farther inboard or outboard.

A close diameter result can give false confidence when the new tire is wider, has a different load index, sits on a different rim width, or will be used on an all-wheel-drive system that expects closely matched rolling circumference.

Technical Details:

Tire comparison math converts every size into a common set of dimensions. Metric sidewall codes are calculated from width, aspect ratio, and rim diameter. Flotation codes already provide diameter and width in inches, so their sidewall height is inferred from the difference between advertised tire diameter and rim diameter.

Once both tires are expressed in millimeters and inches, the comparison uses ratios. Circumference ratio controls speedometer correction. Diameter difference controls ride-height change. Width difference is split per side because a 20 mm wider tire usually adds about 10 mm toward the inside and 10 mm toward the outside before wheel-offset effects are considered.

Formula Core

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

Sidewall = w \times \frac{a}{100}

Diameter = (r \times 25.4) + 2 \times Sidewall

For a flotation size, sidewall height is inferred from the advertised diameter D and rim diameter r, both in inches:

Sidewall = \frac{D - r}{2}

Rolling circumference, speed correction, odometer effect, and ride-height change follow from the two diameters:

C = π \times Diameter

Actual speed = Displayed speed \times \frac{C_{new}}{C_{original}}

Ride-height change = \frac{D_{new} - D_{original}}{2}

For 225/55R17 to 245/50R18, the original diameter is 679.3 mm and the new diameter is 702.2 mm. The diameter change is +22.9 mm, or +3.37%. A 60 mph speedometer reading becomes about 62.0 mph actual, and ride height rises about 11.5 mm.

Review bands and status thresholds for tire size comparison
Setting or status	Boundary	Meaning
Strict OEM	2% diameter band	Close replacement screening for factory-like geometry.
Daily driver	3% diameter band	Normal street-fitment screening for modest changes.
Custom review	5% diameter band	Larger-change screening for deliberate custom, lifted, or off-road use.
All-wheel-drive sensitivity	Effective band is capped at 2%	Rolling-circumference mismatch can matter more on AWD and 4WD systems.
Within review band	Absolute diameter change is less than or equal to the effective band	The geometry passes the selected diameter screen.
Fitment review	Outside the band but less than or equal to 5%	The change needs closer checking before purchase.
Large size change	Greater than 5%	The change is large enough to require strong fitment, speed, and drivetrain review.

Accepted tire size formats and bounds
Input type	Accepted shape	Bounds checked
Metric sidewall code	Examples such as 225/55R17, with optional P, LT, ST, or T prefix.	Width 125 to 405 mm, aspect ratio 20 to 95, rim 10 to 26 in.
Flotation code	Examples such as 33x12.50R15.	Diameter 20 to 44 in, width 5 to 16 in, rim 10 to 26 in, and diameter greater than rim.
Speed reference	Displayed speed in mph or km/h.	Calculated between 5 and 140 mph, or 5 and 220 km/h.
Clearance cushion	Per-side allowance in millimeters.	Compared with half of the section-width change before wheel-offset effects.

Accuracy Notes:

Calculated dimensions are nominal. Real mounted diameter and section width can vary by tire brand, model, rim width, load, inflation pressure, tread depth, and manufacturing tolerance. A worn original tire can also be shorter than its nominal size, so comparing a new proposed tire with a worn tire on the vehicle may exaggerate the apparent change.

Use the vehicle placard, owner's manual, or manufacturer-approved alternate size as the original baseline.
Confirm load index, speed rating, wheel width range, wheel offset, brake clearance, and full-lock steering clearance before purchase.
For AWD and 4WD vehicles, confirm rolling-circumference and tread-depth matching rules with the vehicle manufacturer or a qualified tire dealer.
Test fit when clearance is close, especially with wider tires, lowered suspension, lift kits, tire chains, or aggressive tread patterns.

Worked Examples:

Plus-one sedan fitment

Changing from 225/55R17 to 245/50R18 raises nominal diameter from 679.3 mm to 702.2 mm. The +3.37% diameter change is outside the Daily driver 3% band, so the status moves to Fitment review. At a 60 mph reading, actual speed is about 62.0 mph, and the 20 mm width increase uses a 10 mm per-side cushion.

AWD winter downsizing

A 235/45R18 to 215/55R17 winter change is nearly matched by diameter at -0.06%. The tire is 20 mm narrower overall and adds 12.5 mm of sidewall height, so the speedometer difference at 60 mph is only about -0.04 mph. The all-wheel-drive setting still reminds you to keep all four tires matched in size, model, and wear.

Flotation truck upgrade

A flotation comparison from 31x10.50R15 to 33x12.50R15 increases diameter by 2 in, or +6.45%, and adds 2 in of section width. That becomes a Large size change, with a 60 mph reading near 63.9 mph actual. Use a complete flotation pattern when entering this type of size; missing the rim number will trigger the tire-size warning.

FAQ:

Can I compare metric and flotation tire sizes?

Yes. Both formats are converted into width, sidewall height, rim diameter, overall diameter, circumference, and revolutions per distance before comparison.

Why does a taller tire make the speedometer read low?

The vehicle counts wheel revolutions. A taller tire has more rolling circumference, so each revolution travels farther than the original calibration expects.

Is a result within the review band automatically safe?

No. The review band checks nominal diameter only. Load index, speed rating, wheel width, offset, brake clearance, tire-chain clearance, and manufacturer guidance still matter.

Why does width matter if the diameter is close?

Width affects inner clearance, outer fender position, steering sweep, and wheel compatibility. The fitment check compares half the width change with the selected per-side cushion.

What should I do when the page says to check the tire sizes?

Use a complete sidewall code such as 225/55R17 or 33x12.50R15, then check that the width, aspect ratio, diameter, and rim size are inside normal passenger, truck, or off-road ranges.

Why can real tires differ from the calculated size?

The math uses nominal sidewall markings. Actual mounted size depends on tire model, rim width, inflation pressure, load, tread depth, and manufacturing tolerance.

Glossary:

Aspect ratio: Sidewall height expressed as a percentage of section width in a metric tire size.
Section width: The nominal sidewall-to-sidewall tire width used for clearance checks.
Rolling circumference: The distance a tire travels in one revolution.
Revolutions per mile: The number of tire turns needed to travel one mile.
Review band: The selected diameter-change percentage used to screen a size swap.
Flotation size: An inch-based tire marking such as 33x12.50R15 that starts with advertised overall diameter and section width.

References:

Tire Safety Ratings and Awareness, National Highway Traffic Safety Administration.
How Do I Read My Tire Size On My Sidewall?, Tire Rack.
How Do I Calculate Tire Dimensions?, Tire Rack.