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Tire Size Calculator

Compare two tire sizes and see the change in diameter, width, revolutions per mile and speedometer reading.

Results

MeasureCurrentNewChange

Speedometer check

Speedometer showsActual speed
These are calculated sizes. Real tire measurements vary by brand and tread wear. Always check clearance and your vehicle maker’s approved sizes before changing tires.

Quick Answer

A tire’s overall diameter equals the rim diameter plus two sidewall heights. For a 225/65R17 tire, each sidewall is 5.76 inches, so the overall diameter is 28.52 inches. Changing to 235/60R18 adds about 2.06%, so a speedometer showing 60 mph means you are really doing about 61.2 mph.

Last updated: 2 November 2026 ·How we calculate this: tire size geometry

Table of Contents

How This Tire Size Calculator Works

A tire size such as 225/65R17 has three main numbers: width in millimeters, aspect ratio, and rim diameter in inches. The calculator turns these into real measurements for both tires and shows the difference.

Quick explanation: Overall diameter = rim + 2 × sidewall height.

  • Width: the first number, in millimeters.
  • Aspect ratio: sidewall height as a percentage of width.
  • Rim diameter: the wheel size, in inches.

How to Use the Tire Size Calculator

  1. Find your current tire size on the tire sidewall or the label inside the driver’s door, for example 225/65R17.
  2. Enter its width, aspect ratio and rim diameter as the current tire.
  3. Enter the size you are thinking of fitting as the new tire.
  4. Enter a speed you often drive, then press Calculate.
  5. Check the diameter change and the speedometer table before you buy.

Core Formulas

  • Sidewall height (in): width × aspect ratio ÷ 100 ÷ 25.4
  • Overall diameter (in): rim + 2 × sidewall height
  • Circumference (in): π × overall diameter
  • Revolutions per mile: 63,360 ÷ circumference
  • Actual speed: speedometer reading × new diameter ÷ current diameter

Worked Example

Compare 205/55R16 with 215/45R17.

  • 205/55R16 sidewall = 205 × 55 ÷ 100 ÷ 25.4 = 4.44 in, so diameter = 24.88 in
  • 215/45R17 sidewall = 215 × 45 ÷ 100 ÷ 25.4 = 3.81 in, so diameter = 24.62 in
  • Change = −1.04%, so 60 mph on the speedometer is about 59.4 mph

Real-Life Examples

  • Plus sizing to bigger wheels: going from 215/60R16 to 225/50R17 changes the overall diameter by only -1.14%, so the speedometer stays close. The lower sidewall gives sharper steering but a firmer ride.
  • Taller tires on a truck: moving from 265/70R17 to 285/75R17 adds +7.04% to the diameter. At a real 60 mph, the speedometer would show about 56.1 mph, and the tires may need a lift kit to clear.
  • Winter tires: some drivers fit a narrower size on a smaller rim for snow. The calculator shows whether the diameter stays close to the original.
  • Checking a used car: if the tires fitted do not match the size on the door label, compare the two. You will see how far off the speedometer and odometer are, and whether the size is worth changing back.

Tire Size vs Wheel Size

  • Wheel size: the rim diameter in inches, such as 17 or 18. It must match the last number of the tire size.
  • Tire size: includes the width and sidewall height, which decide the overall diameter.
  • Bigger wheels, same diameter: a larger rim needs a lower aspect ratio to keep the overall size about the same.
  • Ride and cost: lower sidewalls often ride firmer, and large low-profile tires usually cost more.

Common Tire Sizes

Calculated sizes. Real tires vary a little by brand and tread depth.

Tire sizeDiameter (in)Revolutions per mile
195/65R1524.98807
205/55R1624.88811
215/55R1726.31767
225/65R1728.52707
235/65R1830.03672
245/60R1829.57682
265/70R1731.61638
275/55R2031.91632

Before You Change Tire Size

  • Check your door label or manual: these list approved sizes for your vehicle.
  • Clearance: a larger tire can touch suspension or bodywork when turning.
  • Load and speed ratings: a new tire must meet your vehicle’s requirements.
  • Size change: ask a tire professional before fitting a size with a noticeably different diameter.

Common Mistakes to Avoid

  • Mixing sizes on one axle. Both tires on an axle should be the same size.
  • Changing the diameter too much. Many tire shops try to stay within about 3% of the original diameter.
  • Ignoring load and speed ratings. The new tire must meet or beat what your vehicle needs.
  • Forgetting the speedometer. A bigger tire makes it read low, so you may be going faster than you think.
  • Skipping a clearance check. Turn the wheel fully and check the suspension travel before driving.

Frequently Asked Questions

In 225/65R17, 225 is the width in millimeters, 65 is the aspect ratio, R means radial construction, and 17 is the rim diameter in inches.

Multiply width by aspect ratio, divide by 100, then by 25.4 to get sidewall height in inches. Double it and add the rim diameter.

Yes. A larger diameter means the speedometer reads lower than your real speed. A smaller diameter makes it read higher.

It is the sidewall height as a percentage of the tire’s width. A 65 aspect ratio means the sidewall is 65% of the width.

Sometimes, but you should check your vehicle maker’s approved sizes and ask a tire professional. Clearance, load rating and speed rating all matter.

Many tire shops aim to keep the overall diameter within about 3% of the original. Always check your vehicle maker’s approved sizes.

It can. Larger, heavier or wider tires often add rolling resistance and weight, which can raise fuel use.

It is printed on the tire sidewall and on the tire label inside the driver’s door or in the owner’s manual.

References & Sources

SourceUsed ForLink
Bridgestone Americas, determining tire size Meaning of tire size numbers. https://www.bridgestoneamericas.com/en/company/safety/choosing-tires/determining-tire-size
Modern Tire Dealer, understanding tire dimensions Aspect ratio definition. https://www.moderntiredealer.com/articles/17231-understanding-tire-dimensions