Tire Size Calculator
Compare two tire sizes: diameter, circumference and speedo error.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- New overall diameter
- 28.57 in
- Original diameter
- 28.52 in
- Diameter change
- 0.06 in (0.21%)
- Circumference (new)
- 89.77 in
- Revolutions per mile
- 706
- Speedo at indicated 60 mph
- 60.1 mph actual
- Fitment
- Within the usual ±3% safe window
Quick answer
A tire's overall diameter is the rim diameter plus two sidewalls, where each sidewall is the width (mm) × aspect ratio ÷ 100, converted to inches by dividing by 25.4. A 225/65R17 is 17 + 2 × (225 × 0.65) ÷ 25.4 = 28.52 in tall. Most guidance keeps a replacement within ±3% of the original diameter.
What the Tire Size calculator does
This calculator compares two tire sizes, typically the size on your vehicle now and one you are considering. It shows each tire's overall diameter, the change in inches and percent, the new tire's circumference and revolutions per mile, and how fast you will really be going when the speedometer reads 60 mph.
Use it before fitting larger off-road tires, moving to bigger wheels (plus-sizing) or substituting a size when your exact one is out of stock. A taller tire makes the speedometer read low and slightly raises effective gearing; a shorter one does the opposite. For the speedometer effect on its own, see the speedometer error calculator.
The figures are nominal dimensions from the size code. Actual tires vary a little by brand and tread depth, and diameter is only one part of fitment: width, offset, load index and speed rating all matter too.
The formula
Sidewall (in) = width × aspect ratio ÷ 100 ÷ 25.4 Diameter = rim + 2 × sidewall Circumference = π × diameter Revolutions per mile = 63,360 ÷ circumference Actual speed = indicated speed × new diameter ÷ original diameter
| Symbol | Meaning | Unit |
|---|---|---|
| width | Section width, the first number in the size code (225 in 225/65R17) | mm |
| aspect ratio | Sidewall height as a percentage of width (65 in 225/65R17) | % |
| rim | Wheel diameter the tire fits (17 in 225/65R17) | in |
| 25.4 | Millimeters per inch | mm/in |
| 63,360 | Inches in one mile | in |
This is the standard geometry for metric (P-metric and Euro-metric) tire size codes, as standardized by the Tire and Rim Association and ETRTO. Flotation sizes used on some off-road tires (for example 33×12.50R17) state the overall diameter directly in inches, so you can compare them by entering a metric size with the same diameter or by reading the first number as the diameter.
Worked examples
Slightly wider replacement on an SUV
An SUV owner wants to replace 225/65R17 tires with 245/60R17 for a wider footprint.
Inputs
- Original width225 mm
- Original aspect ratio65 %
- Original rim17 in
- New width245 mm
- New aspect ratio60 %
- New rim17 in
Result
- New overall diameter28.57 in
- Original diameter28.52 in
- Diameter change0.06 in (0.21%)
- Circumference (new)89.77 in
- Revolutions per mile706
- Speedo at indicated 60 mph60.1 mph actual
The diameter changes by only about 0.2%, so the speedometer stays accurate. The lower aspect ratio offsets the extra width, which is the usual way to go wider without going taller.
Bigger tires on a lifted pickup
A truck on 265/70R17 is moving to 285/75R17 all-terrain tires after a lift.
Inputs
- Original width265 mm
- Original aspect ratio70 %
- Original rim17 in
- New width285 mm
- New aspect ratio75 %
- New rim17 in
Result
- New overall diameter33.83 in
- Original diameter31.61 in
- Diameter change2.22 in (7.04%)
- Circumference (new)106.28 in
- Revolutions per mile596
- Speedo at indicated 60 mph64.2 mph actual
About 7% taller: at an indicated 60 mph you would really be doing about 64 mph, and the odometer will under-record mileage. Plan to recalibrate the speedometer and check clearance at full steering lock and suspension compression.
Plus-two wheel upgrade
A compact car on 205/55R16 moving to 18-inch wheels with 225/40R18 tires.
Inputs
- Original width205 mm
- Original aspect ratio55 %
- Original rim16 in
- New width225 mm
- New aspect ratio40 %
- New rim18 in
Result
- New overall diameter25.09 in
- Original diameter24.88 in
- Diameter change0.21 in (0.84%)
- Circumference (new)78.81 in
- Revolutions per mile804
- Speedo at indicated 60 mph60.5 mph actual
Under 1% change in diameter, well within the usual window. The thinner sidewall sharpens steering response but rides more firmly and leaves the wheel rim more exposed to pothole damage.
How to read a tire size code
Take P225/65R17 102H as an example:
- P: passenger tire (P-metric). LT means light truck; no letter means Euro-metric.
- 225: section width in millimeters.
- 65: aspect ratio, so the sidewall is 65% of 225 mm, about 146 mm.
- R: radial construction.
- 17: rim diameter in inches.
- 102: load index, the maximum load per tire (102 is 850 kg, about 1,874 lb).
- H: speed rating (H is 130 mph, 210 km/h).
The recommended size and pressures for your vehicle are on the placard inside the driver's door jamb.
What changes when you go up or down a size
The percent change from the calculator applies to everything that depends on wheel revolutions. A 3% taller tire means 3% more distance per turn, so the speedometer, odometer and effective final-drive ratio all shift by about 3%.
| Effect | Larger diameter | Smaller diameter |
|---|---|---|
| Speedometer | Reads lower than true speed | Reads higher than true speed |
| Odometer | Under-records distance | Over-records distance |
| Acceleration | Slightly slower (taller effective gearing) | Slightly quicker |
| Highway RPM | Lower at a given speed | Higher at a given speed |
| Ground clearance | Increases by half the diameter change | Decreases by half the diameter change |
| Measured fuel economy | Appears worse if calculated from odometer miles | Appears better if calculated from odometer miles |
Staying inside the ±3% guideline
Keeping the overall diameter within about 3% of the original is a widely used rule of thumb among tire retailers and vehicle makers. Beyond that, speedometer error grows, and ABS, stability control and automatic transmission shift points, which rely on wheel-speed sensors, are working from figures the vehicle was not calibrated for.
Also check that the new tire's load index and speed rating are at least as high as the original, and that the wheel width is within the tire maker's approved rim-width range. The RPM to speed calculator shows how a new diameter changes engine speed at highway cruise.
How to use the Tire Size Calculator
- 1Read the current size from the tire sidewall or door-jamb placard, for example 225/65R17.
- 2Enter the original width (mm), aspect ratio (%) and rim diameter (in).
- 3Enter the same three numbers for the tire you are considering.
- 4Check the percent change and the fitment line: under ±3% is the usual safe window.
- 5Use the speedometer line to see your true speed at an indicated 60 mph.
Frequently asked questions
How much bigger can I go on tire size?
Most guidance keeps overall diameter within about 3% of stock without other changes. Larger jumps usually need a lift or leveling kit, clearance checks and speedometer recalibration.
Will bigger tires make my speedometer wrong?
Yes. A tire 3% taller covers 3% more ground per revolution, so at an indicated 60 mph you are really doing about 61.8 mph.
Can I put a wider tire on my stock rim?
Often by 10–20 mm, as long as the rim width is within the tire maker's approved range and the tire clears the suspension and fenders. Check the manufacturer's rim-width chart for the exact size.
What does 265/70R17 mean in inches?
It is about 31.6 in tall and 10.4 in wide, fitting a 17-in wheel. The sidewall is 265 × 0.70 = 185.5 mm, or 7.3 in.
Do bigger tires affect gas mileage?
Usually they lower it slightly through extra weight and rolling resistance. A taller tire also makes the odometer under-record miles, so if you work out fuel economy with the MPG calculator, correct the trip-meter distance first.
References
- Tire and Rim Association Year Book — The Tire and Rim Association, Inc.
- Tire Safety: Everything Rides On It — National Highway Traffic Safety Administration
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
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