Quick answer
Tire diameter difference = new diameter ÷ old diameter − 1, and actual speed = indicated speed × diameter ratio. Replacing a 225/45R17 (634.3 mm) with a 245/45R17 (652.3 mm) makes the tire 2.84% taller, so an indicated 60 means about 61.7.
Enter the old and new tire sizes. The calculator compares their overall diameters and the speedometer error. Use this tire size comparison calculator to check your own figures.
Related tools: Tire Size Calculator: Overall diameter, sidewall height and revolutions per mile from a tire size such as 225/45R17. Pizza Size Comparison Calculator: Which pizza gives more area per dollar. Carburetor Size Calculator: Carburetor airflow needed from displacement, peak rpm and volumetric efficiency.
How to use the Tire Size Comparison Calculator
- Enter old tire: Section width (mm) (above 0).
- Enter old tire: Aspect ratio (%) (above 0).
- Enter old tire: Rim diameter (in) (above 0).
- Enter new tire: Section width (mm) (above 0).
- Enter new tire: Aspect ratio (%) (above 0).
- Enter new tire: Rim diameter (in) (above 0).
- Enter indicated speed (above 0). It starts at 60.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Old tire: Section width | A number, in mm | > 0 |
| Old tire: Aspect ratio | A number, in % | > 0 |
| Old tire: Rim diameter | A number, in in | > 0 |
| New tire: Section width | A number, in mm | > 0 |
| New tire: Aspect ratio | A number, in % | > 0 |
| New tire: Rim diameter | A number, in in | > 0 |
| Indicated speed | A number | > 0, starts at 60 |
Formula and method
Formula
error = new diameter ÷ old diameter − 1; actual speed = indicated × diameter ratio
IEEE-754 double precision; shown to the decimals indicated.
Assumes the speedometer was accurate with the old tires. Speed is in whatever unit you enter.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Old tire: Section width = 225; Old tire: Aspect ratio = 45; Old tire: Rim diameter = 17; New tire: Section width = 245; New tire: Aspect ratio = 45; New tire: Rim diameter = 17; Indicated speed = 60.
| Result | Value |
|---|---|
| Diameter difference | 2.84% |
| Diameter of the old tire | 634.3 mm |
| Diameter of the new tire | 652.3 mm |
| Actual speed at the indicated speed | 61.7 |
| Speedometer error | 2.84% |
Example 2
Inputs: Old tire: Section width = 205; Old tire: Aspect ratio = 55; Old tire: Rim diameter = 16; New tire: Section width = 225; New tire: Aspect ratio = 50; New tire: Rim diameter = 17; Indicated speed = 100.
| Result | Value |
|---|---|
| Diameter difference | 3.94% |
| Diameter of the old tire | 631.9 mm |
| Diameter of the new tire | 656.8 mm |
| Actual speed at the indicated speed | 103.94 |
| Speedometer error | 3.94% |
Example 3
Inputs: Old tire: Section width = 265; Old tire: Aspect ratio = 70; Old tire: Rim diameter = 17; New tire: Section width = 285; New tire: Aspect ratio = 70; New tire: Rim diameter = 17; Indicated speed = 55.
| Result | Value |
|---|---|
| Diameter difference | 3.49% |
| Diameter of the old tire | 802.8 mm |
| Diameter of the new tire | 830.8 mm |
| Actual speed at the indicated speed | 56.92 |
| Speedometer error | 3.49% |
The first line is the headline result (diameter difference). Below it the tire Size Comparison Calculator lists diameter of the old tire, diameter of the new tire, actual speed at the indicated speed and speedometer error, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Why width changes diameter
With the same aspect ratio, a wider tire has a taller sidewall: 245 × 0.45 = 110.25 mm against 101.25 mm. Two sidewalls add 18 mm to the height, which is the 2.84% difference.
Staying within tolerance
Many sources advise keeping the overall diameter within about 3% of the original to protect speedometer accuracy and gearing, and checking clearance. A positive error means you travel faster than the speedometer shows.
How the result changes with old tire: Section width
Holding every other input at the first worked example, this table changes only old tire: Section width. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Old tire: Section width | Diameter difference | Diameter of the old tire | Diameter of the new tire |
|---|---|---|---|
| 112.5 | 22.37% | 533.1 mm | 652.3 mm |
| 225 | 2.84% | 634.3 mm | 652.3 mm |
| 450 | -22.05% | 836.8 mm | 652.3 mm |
| 900 | -47.47% | 1,241.8 mm | 652.3 mm |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Old tire: Section width: entering "0" shows "Old tire: Section width must be greater than 0." beside that field and no result.
- Indicated speed: entering "0" shows "Indicated speed must be greater than 0." beside that field and no result.
Choosing between the Tire Size Comparison Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Tire Size Calculator](/automotive/tire-size-calculator/) | section width, aspect ratio and rim diameter | Overall diameter |
| [Pizza Size Comparison Calculator](/lifestyle/pizza-comparison/) | pizza 1 diameter, pizza 1 price and pizza 2 diameter | Better value |
| [Carburetor Size Calculator](/automotive/carburetor-size-calculator/) | displacement, peak engine speed and volumetric efficiency | Required airflow |
| [Sample Size Calculator](/statistics/sample-size-calculator/) | estimate a, confidence level and margin of error | Required sample size |
| [Portion Size Calculator](/cooking/portion-size-calculator/) | total amount of food and people | Per person |
Background: vehicle sizing and speed
A taller tyre travels farther per revolution, so the speedometer reads low and the effective gear ratio becomes numerically lower; a shorter tyre does the opposite. True speed is roughly the speedometer reading times the new tyre diameter divided by the old one. Verify with GPS over a known distance before changing parts.
Assumptions and limitations
- A positive error means you travel faster than indicated
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for old tire: Section width and indicated speed to confirm this.
Frequently asked questions
Will a taller tire reduce my acceleration?
Slightly, because it acts like a taller final-drive gear.
How do I read the tire code?
Width in mm, aspect ratio in percent, then R and the rim diameter in inches.
Does it affect the odometer?
Yes, by the same percentage.
What does the Tire Size Comparison Calculator calculate?
Compare two tire sizes and the resulting speedometer error. It returns Diameter difference, Diameter of the old tire, Diameter of the new tire, Actual speed at the indicated speed and Speedometer error.
What formula does the Tire Size Comparison Calculator use?
error = new diameter ÷ old diameter − 1; actual speed = indicated × diameter ratio. Assumes the speedometer was accurate with the old tires.
What do I need to enter in the tire Size Comparison Calculator?
You enter Old tire: Section width (mm), Old tire: Aspect ratio (%), Old tire: Rim diameter (in), New tire: Section width (mm), New tire: Aspect ratio (%), New tire: Rim diameter (in) and Indicated speed.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. A positive error means you travel faster than indicated.
What is an example of a tire Size Comparison result?
With old tire: Section width 225, old tire: Aspect ratio 45, old tire: Rim diameter 17, new tire: Section width 245 and new tire: Aspect ratio 45, the diameter difference is 2.84%.
Related calculators
- Tire Size Calculator — Overall diameter, sidewall height and revolutions per mile from a tire size such as 225/45R17.
- Pizza Size Comparison Calculator — Which pizza gives more area per dollar.
- Carburetor Size Calculator — Carburetor airflow needed from displacement, peak rpm and volumetric efficiency.
- Sample Size Calculator — Sample size for estimating a mean or a proportion.
- Engine Displacement Calculator — Displacement from bore, stroke and number of cylinders.
- Height Comparison Calculator — Difference and ratio between two heights.
Sources
- OpenStax, Prealgebra 2e (Rice University): fractions, decimals, percents, ratios and rounding
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international inch 25.4 mm, avoirdupois pound 0.45359237 kg)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.