Tire Size Calculator – Compare Sizes

We created the Tire Size Calculator so you can calculate your tires\' dimensions and make comparisons to understand the effects of changing your tyre size. It will help you change old tyres, upgrade wheels or switch to larger tyres.

In the USA it is common to have tyre sizes such as 225/60R17.

The numbers & letters will to represent the size of the tyre with their meaning;

225 - Tyre width in millimeters.

60 - Aspect ratio.

R - Radial tire type construction, commonly used to construct tyres for modern passenger vehicles.

17 - Wheel size in incheidence.

For example, the size of a tyre such as the tyre above will be,

Tyre width - 225mm.

Aspect ratio - 60%.

Fits a 17\" wheel.

This will help give us the overall dimension in the following areas;

Tyre diameter.

Tyre sidewall.

Millimeters travelled around the tyre.

Revolutions per mile.

With our Tire Size Calculator it will help you make preferred comparisons of your current tyre size with tyres of another size to see,

Any differences in overall diameter.

Overall tyre width.

Tyre SID.

Millimeters travelled around the tyre.

Estimated speedometer difference.

Whether you have a Sedan, SUV, Pickup Truck, Sports Car or Crossover vehicle, this Tire Size Calculator will help you understand how changing tyre size for an alternative option will affect matters to aid your decision. Traditional users of this calculator are people interested in changing or upgrading their wheels with manufacture wheels, plus sizing, off-road tyres or to meet original specifications.

How to Use

Using the Tire Size Calculator is very simple. Enter the details on your current tyre and the new option.

Click calculate below after entering,

Origional tyre width - Enter the measured width in mm of your old tyre. ie. 225.

Origional tyre aspect ratio - Enter the aspect ratio of your tyre. ie. 60.

Origonal wheel diameter - Enter the wheel size in Inches. ie 17.

New tyre width - Add your alternative tyre width.

New tyre aspect ratio - Add your new tyre aspect ratio.

New wheel diameter - Add your new replacement parts wheel diameter in Inches.

Button below.

Results

Overall diameter difference. Tyre/SID height. How many millimeters are travelled around it. And Speedometer difference.

You can experiment by edditing different tyre sizes to get a close match to the original tyre specifications.

Formula

To calculate the overall tire size of the tyre;

Tyre Sidewall:

[math]SH = \\frac{W X AR}{100}[/math]

Where,

SH = Sidewall height in mm. W = Width. AR = Aspect Ratio in %.

Wheel diameter:

[math] WD=R X 25.4 [/math]

Where, R = Wheel diamter in inches. WD = Wheel diameter in mm.

Overall tyre diameter:

[math]D = (2 X SH) + WD [/math]

Tyre Circumference:

[math]C = \\pi \\ X D[/math]

Difference in diameter in %:

[math]Difference (%) = \\frac{(D_new - D_old)}{D_old} X 100[/math]

Below we will show you how to calculate a tyre size example.

Old tyre size: 225/60R17. New or alternative tyre: 235/55R18.

Check tyre differences.

Old tyre:

Width = 225mm.

Aspect ratio = 60%.

Wheel size = 17\".

Work out tyre SID:

[math]SH = \\frac{225 X 60}{100} = 135mm.[/math]

Convert wheel diameter:

[math]17 X 25. 4=431.8 mm[/math]

Work out overall tyre diameter.

[math]D = (2 X 135) + 431.8 [/math].

[math]D=701.8mm [/math].

New tyre:

Tyre width = 235mm. Aspect = 55%. Wheel diameter = 18 inches.

Work out tyre SID:

[math]SH = \\frac{235 X 55}{100}= 129.25mm[/math].

Convert wheel diameter:

[math]18 X 25. 4=457.2 mm[/math].

Work out overall tyre diameter:

[math]D = (2 X 129.25) + 457.2[/math].

[math]D=715.7mm[/math].

Results:

Width Aspect Ratio Wheel Diameter Tyre SID Overall diameter Circumfearence Overall Diameter

225mm 60% 17\" 135mm 701.8mm 2204.8

235mm 55% 18\" 129.25mm 715.7 Assumption. Its shows that there is around 13.9mm more than the original tyre which is around a 1.98% increase.

Means your tyre will travel less miles than the origional tyre which could have an effect on your speedometer.

Understanding Results.

Once you enter the details to your tyre the calculator returns the following.

• Tyre width: Nominal width of the tire measured from SID to SID.
• Aspect ratio: True representation of SID as a % of width. A low aspect ratio will be smaller SID.
• SID: Distance from wheel rim to outer tyre. Calculated by multiplying the tyre width and aspect ratio.
• Overall Dia.: approximation of the overall tire height including the wheel & SID.
• Circumferance: Measurement of the distance travelled of the tyre per full revolution. Based on the calculated unloaded diameter.
• Diameter Diff: Shows how big the alternative tyre is compared to origional tyre.
• Spedometer Diff: Changing your tyre diameter may change how many times your wheels rotate vs miles undertaken. Large tyres will Travel more distance compared to original tyre per rotation.

Understanding these measurements will give you insight into whether your Alternative tyre will suit your vehicle.

Factors that Affect tyre size

Tyre width.

Larger tyres may help to have a larger contact patch depending on protection & tyre width. However may affect how much rolling resistance, responsiveness under steering & clearance.

Aspect Ratio.

A low ratio means a smaller SID. Low profile tyres can provide more responsive handling but may be less forgiving on rough surfaces.

Wheel Diameter.

Changing wheel diameter will effect the tyres size unless you adjust the aspect ratio.

Overall Tyre diameter.

Large tyre diameters will change the clearance on the ground. The gear ratios, overall and spedometer will also change.

Tyre Pressure.

Incorrect tyre pressure will change how your tyre appears, affecting Handling, braking and Tyre wear. Ensure you follow the manufacturers Pressure Guidelines.

Vehicle Specifications.

All vehicles will have tyre size recommendations based on Suspension Specifications, Clearance, Load capability & Handling.

Tyre Type & Manufacturer.

Tyres manufacturer can have differing overall sizes.

Benefits & Practical applications.

Compare your new tyres.

See how much your spedometer will change.

Upgrade wheels from 16\" to 17\" or 18\".

Understand the change in ground clearance.

SUV / pickup owners can compare larger tyres for off road upgrades.

It helps you understand what will happen to SID, tyre width and overall tyre diameter.

Save time. Ill need to do multiple mathematical formula.

Accuracy & Assumptions.

This calculator uses standard tyre formulas, to help you. Agruably these may not have the correct dimensions due to.

Nominal tyre sizes. Calculations will use theoretical values.

Different manufacturers will have differing tyre diameters and widths.

Tyre Pressures can vary loaded radius of tyre. Also if Vehicle is highly loaded.

Wear from treads will reduce overall tire diameter over time.

No automatic check of wheel width fitment & Suspension clearance.

Tyre size changes may affect ABS, TrACTION CONTROL.

Spedometer. May based off original spedometer settings.

Ensure to check the Tyre load capability, speed & manufacturers guidelines.

Always refer back to the vehicle owners handbook, Tyre spec & speak to a professional tyre specialist to ensure chosen tyres cater to vehicles loaded & speed requirements. As well as ensuring tyre clearances will work with normal driving & suspension setup.

Frequently Asked Questions

Q1: What does a tyre size such as 225/60R17 mean.

This tyre size means your tyre will measure 225mm wide and your tyre\'s SID width will be 60% of the width. It will fit onto a 17inch wheel. The R Letter indicated Radial tyre type such as used with most passenger vehicles.

Q2: How to calculate tyres overall diameter.

Multiply the Tyre width by Aspect Ratio then divide by 100 to calculate the tyre SID. Mulptiply the wheels diameter by 25.4 to convert it into mm. Then use the equation (2 x tyre SID) + wheel diameter.

Q3: Will my spedometer change if I change my tyre sizes.

Yes. Chagning your tyre diameter will make changes to your spedometer accuracy. Larger tyres will mean your vehicle travels a further distance per rotation. If your tyre is not recalibrated it can make your spedometer show a lower speed than actually traveling.

Q4: What is the difference in tyre width & aspect ratio.

Tyre width is measured in mm whilst aspect ratio is measured in % of how much of the Tyre width is made up of SID. E.G: Tyre with a 225/60 would have a SID height of 135mm.

Q5: Can I fit larger tyres.

You may but it depends on wheel width, Suspension Clearance, Fender space, Load capabilities & Manufacturers guidelines.

Q6: What is the difference between 17inch & 18inch tyres.

These are wheel sizes. The overall tyres may differ so you will need a tyre suitable for an 18inch rim. The overall tyre will depend upon wheel diameter and SID height.

Q7: What difference in tyre size is acceptable.

Many vehicle owners recommend a 3% difference or under, however this is rarely a universal Safety spec. Ensure always to check the Tyre manufacturers guidelines and you vehicles specifications to ensure tolerances, tyce clearance, loading & electronic requirements.