Tire Size, Revolutions & Speedometer Comparison

Compare nominal diameter, circumference and wheel revolutions for two tire sizes.
See theoretical speed differences with unchanged calibration and prepare an installer checklist.

Compare two tire sizes

Enter width (mm) / aspect ratio (%) R rim (inches), such as 205/55R16. R, ZR and spaces are supported. Leave out load and speed ratings. Default sizes are examples, not a recommendation for your vehicle.

Original / reference size

Candidate size

Indicated speed to compare (km/h)

0–300 km/h, up to one decimal place. Not a recommended driving speed.

Manufacturer approval for both sizes

Supported notation and input limits

Supported: integer width 100–455 mm, integer aspect ratio 20–85%, and rim diameter 10–24 inches in 0.5-inch steps. These application limits do not establish product availability or fitment approval. P/LT prefixes, flotation sizes such as 31x10.50R15, and load/speed suffixes are unsupported.

Manufacturer approval not checked — review the vehicle manual and approved fitments.

Diameter change

+0.38%

205/55R16 → 225/45R17

Diameter difference

+2.4 mm

Candidate − original

Geometric radius difference

+1.2 mm

Actual ride-height change may differ

Theoretical road speed from indicated speed

100.0 → 100.38 km/h

Difference: +0.38 km/h

Assumes the original indicated speed equals road speed and calibration remains unchanged after replacement. This does not measure actual vehicle speed or establish a speeding margin.

Conversely, at a fixed road speed of 100.0 km/h, theoretical candidate indication is 99.62 km/h.

Dimensions and wheel revolutions

Nominal dimensions and revolutions for original and candidate tires
Measure	205/55R16	225/45R17
Width (mm)	205.00	225.00
Aspect ratio (%)	55.00	45.00
Rim (inches)	16.00	17.00
Sidewall height (mm)	112.75	101.25
Diameter (mm)	631.90	634.30
Circumference (mm)	1,985.17	1,992.71
Revolutions per kilometer (rev/km)	503.73	501.83
Wheel rpm at same road speed (100.0 km/h)	839.56	836.38

Revolutions per distance change: -0.38%. The rpm row compares wheels at the same assumed road speed, not engine rpm.

Comparison across indicated speeds

Candidate theoretical road speeds with unchanged original calibration
Indicated km/h	Theoretical road km/h	Difference km/h
30	30.11	+0.11
50	50.19	+0.19
80	80.30	+0.30
100	100.38	+0.38
120	120.46	+0.46

Confirm with the installer

Manufacturer approval for the model year, trim and front/rear axle
Rim width, offset, brake/body clearance, load index and speed rating
Inflation pressure, ABS/ESC/AWD requirements and any vehicle setting changes

Nominal geometry does not certify fitment, inspection or legal compliance. The report includes the approval status and assumptions.

Compare tire dimensions before placing an order

This tire size calculator compares the nominal geometry of an original size and a candidate size.
Wheel diameter alone does not tell you how large the complete tire will be.
Width and aspect ratio determine the sidewall height, which contributes to overall diameter, circumference and revolutions per kilometer.

Use the comparison when considering replacement tires or an alternative wheel package on the vehicle manufacturer’s approved list.
The result helps organize dimensions before a purchase; it does not identify an approved product or certify a particular tire and wheel combination.
Keep the vehicle manual alongside the comparison to reduce ordering mistakes.

Your next step

Save the comparison and ask the installer to verify approval for the exact model year, trim, wheel and axle.
Once the specification is confirmed, compare prices using the same product, quantity and service scope.
The tire rental versus purchase calculator covers that separate cost decision.

Read the three numbers in 205/55R16

205: nominal width

The nominal section width is measured in millimeters.
It is not a direct measurement of tread contact width or outward protrusion after installation.

55: aspect ratio

Sidewall height is expressed as a percentage of width.
Multiply the width by 0.55; the number does not mean a 55 mm sidewall.

16: rim diameter

The matching rim diameter is measured in inches.
R describes radial construction, not radius.

Load index and speed rating following the size are separate specifications.
Enter only the core size and check the remaining markings with the installer.
A tire with a different rim diameter requires a matching rim; the comparison does not suggest installing it on the original rim.

Inputs and supported formats

Check the manual and current sidewall before entering the original size.
If the installed tires differ from the original equipment, establish which size the speedometer calibration uses.
Enter the candidate and check the width, aspect ratio and rim values in the table.
Incomplete or unsupported notation pauses the result instead of keeping a stale comparison.
Enter an indicated speed in km/h.
The input range is a mathematical comparison range, not a speed recommendation or road limit.
Record whether both sizes are approved for the vehicle, axle and wheel.
Editing or swapping sizes resets this declaration to not checked.
Read the diameter, revolutions and speed tables, then save the TXT report.
The report includes the approval declaration and calculation assumptions.

Application limits

Width must be an integer from 100–455 mm, aspect ratio an integer from 20–85%, and rim diameter 10–24 inches in 0.5-inch steps.
Indicated speed accepts 0–300 km/h with one decimal place.
Letter case, spaces around separators, R and ZR are supported.
P/LT prefixes, load/speed suffixes and flotation notation such as 31x10.50R15 are unsupported.
These limits do not imply that every combination exists or is approved.

Diameter, circumference and wheel revolutions

Let W be width, A aspect ratio, and R rim diameter.
One inch is 25.4 mm.
Calculations retain full precision; only displayed values are rounded, so multiplying numbers copied from the screen may differ slightly in the final digits.

Nominal geometry

Sidewall H = W × A / 100
Diameter D = 25.4 × R + 2 × H
Circumference C = π × D
H, D and C are in mm

Wheel revolutions

Revolutions per km N = 1,000,000 / C
At the same road speed V: rpm = V × N / 60
V is in km/h; N is in rev/km

The model assumes one wheel revolution covers one nominal circumference.
These are wheel revolutions, not engine rpm or transmission ratios.
A loaded tire does not necessarily roll one nominal geometric circumference per turn; distinguish manufacturer rolling data from this calculation.

What does the speed comparison assume?

Original indication equals road speed; calibration stays unchanged

For original diameter Di and candidate diameter Dn, the ratio is q = Dn / Di.
At indicated speed V, candidate theoretical road speed is V × q, with a difference of V × (q − 1).
Conversely, at fixed road speed V, theoretical candidate indication is V / q.
These outputs hold different quantities fixed and must not be interchanged.

A larger diameter covers more distance per wheel revolution, increasing theoretical road speed for the same indication.
A smaller diameter reverses that direction.
Original speedometer bias, actual rolling circumference and any recalibration are excluded, so the result cannot establish whether your real vehicle is traveling above or below its indication.
Do not use it for speedometer correction or as a margin for avoiding speed enforcement.

Worked example: 205/55R16 → 225/45R17

These default sizes illustrate the calculation, not an approved pairing for a particular vehicle.
Replace them with the sizes you are checking.

Original and candidate nominal dimensions in the worked example
Measure	205/55R16	225/45R17
Sidewall mm	112.75	101.25
Diameter mm	631.9	634.3
Circumference mm	1,985.17	1,992.71
Revolutions rev/km	503.73	501.83
Wheel rpm at assumed road speed 100km/h	839.56	836.38

Diameter increases by +2.4mm, radius by +1.2mm and diameter percentage by approximately +0.38%.
At an indication of 100km/h, theoretical road speed is 100.38km/h.
At a fixed road speed of 100km/h, theoretical indication is 99.62km/h.
Revolutions per distance decrease by approximately 0.38%.

Check the sign and the reference size

Increases and decreases

Differences are candidate minus original.
Positive diameter means the candidate is larger; a negative revolutions change means fewer turns per distance.
Diameter and revolutions changes use reciprocal ratios, so their percentage magnitudes are not exactly equal.

Swapping changes the denominator

Swapping sizes reverses the millimeter difference but also changes the percentage denominator.
Use the swap button instead of merely reversing a percentage sign.
Identical sizes produce zero dimension, revolutions and speed differences.

The geometric radius difference is half the diameter difference.
It does not guarantee the same real ground-clearance change because deflection, load and suspension conditions are not modeled.
A small number does not establish brake or body clearance.

Practical uses: replacement, wheel changes and quotes

Compare approved wheel packages

If the manufacturer lists multiple fitments, compare their complete sizes.
For staggered setups, compare each axle separately.
This calculator does not validate front/rear combinations or determine an AWD tolerance.

Check an online order

Read beyond the advertised wheel size.
Even with the same rim diameter, different widths or aspect ratios change overall diameter.
Compare the parsed dimensions with the manual, and check product identity and service ratings as separate order details.

Prepare for an installer consultation

Send the comparison together with unresolved questions.
Ask about rim width, offset, hub and brake clearance, plus any required vehicle settings before confirming the order.
The report does not calculate installation charges or establish return terms.

Limits and checks before installation

Nominal geometry does not guarantee measured diameter or dynamic rolling circumference.
Pressure, load, wear and product construction are not inputs; this is not a precise new-versus-worn comparison.
No percentage difference produces a safe-fitment verdict.
Do not apply a rule of thumb universally across vehicles and countries.
Three dimensions cannot verify load index, speed rating, wheel compatibility, clearance or vehicle control-system requirements.
Selecting approval checked records your declaration; it does not query a vehicle database.
The speed table applies one diameter ratio to several indications.
It is not a road test, inspection certificate, insurance decision or legal approval.

Frequently asked questions

Does a larger wheel always mean a larger tire diameter?

No.
Overall diameter includes both sidewalls as well as the rim.
A larger rim with shorter sidewalls can produce a similar total diameter.

Is a small diameter difference safe to install?

Diameter alone cannot establish fitment.
Check manufacturer approval, wheel compatibility, clearance, load index and speed rating.
The calculator has no safe percentage threshold.

Does this show my exact actual speed?

No.
Theoretical speed assumes the original indication equals road speed and calibration remains unchanged.
Original bias and actual rolling circumference are excluded, so this is not a speed measurement or calibration tool.

Are wheel revolutions the same as engine rpm?

No.
Engine rpm also depends on transmission and final-drive ratios.
This tool calculates wheel revolutions only.

What happens at zero speed?

Theoretical speed and wheel rpm become zero.
Diameter, circumference, revolutions per distance and diameter ratio remain properties of the entered sizes.

Can I include P, LT or service ratings?

Only the width/aspect R rim or ZR core notation is supported.
Unsupported prefixes and suffixes pause the result.
Using core dimensions does not verify the product type or service ratings.

How do I compare staggered front and rear tires?

Compare the original and candidate for each axle separately.
Front/rear compatibility and AWD tolerances require the vehicle manufacturer’s instructions.

Does selecting approval checked certify the result?

No.
It records that you checked the relevant documents.
The tool does not query a VIN or approval database, and changing sizes resets the declaration.

Sources and review date

Sources checked on 2026-09-22; manufacturer guidance should be reviewed every six months.
The geometry is country independent.
For Korean context, the National Law Information OPEN API returned the Rules on the Performance and Standards of Motor Vehicles and Motor Vehicle Parts, effective 2026-07-10, law ID 007939, MST 288083.
Articles 12, 54 and 110 concern running gear, speedometers/odometers and speedometers respectively.
Those requirements are not converted into a tire-diameter tolerance or a legal pass/fail result here.

The Continental bulletin is older technical guidance, not a current vehicle approval list.
Use current vehicle specifications and installer verification for your particular combination.
The Korean legal context is not a statement of approval rules in other countries.

Prepare the comparison before confirming your order

Check both sizes once more and save the report.
Bring the dimensions, assumptions and unresolved fitment questions to the installer.
After choosing an approved specification, compare costs using matching products and service terms.

Compare tire sizes again Tire rental versus purchase costs