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Tire Size Comparison Calculator

Compare tire diameters, sidewall changes, and estimated speedometer error.

Fitment

Compare tire diameter, sidewall, and speedometer change

Useful for checking how a tire size change affects overall diameter and the numbers you see on the dash.

Current tire size

New tire size

Results update as you type

Result summary

Updated after each calculation with the most important output.

Current diameter

28.52 in

New diameter

29.03 in

Diameter difference

0.51 in

Difference %

1.79%

Ride height change

0.26 in

Speedometer error

1.79%

How to use this tool

Tire size changes affect overall diameter first. That, in turn, changes the effective distance traveled per wheel rotation and can shift speedometer readings.

This tool compares dimensions only. It does not guarantee fitment, clearance, suspension compatibility, or safe use on a specific vehicle.

Need a copy of these results?

Step by step

How to use this tool

  1. 1

    Find your current tire size on the sidewall of your tire — it reads like 225/65R17.

  2. 2

    Enter the three numbers: width (225), aspect ratio (65), and wheel diameter (17).

  3. 3

    Enter the new tire size you are considering in the same format.

  4. 4

    Review the diameter difference and speedometer error percentage. Keep the difference under 3% to avoid drivability issues.

Decision context

What this calculator helps you decide

Use Tire Size Comparison Calculator when you need a quick, structured answer before you spend money, approve work, prepare a trip, compare options, or share information with a buyer, seller, shop, lender, or insurer. Enter the inputs you already know, review the result, then use the assumptions and limits below to decide what to check next.

Inputs and outputs

Start with the inputs that most affect this decision: Original Tire Size (e.g., 225/45R17), New Tire Size (e.g., 245/40R18). The output is meant to make the next step easier to compare, not to replace a written quote, inspection, policy document, loan disclosure, or local rule.

The main outputs are Original Diameter, New Diameter, Diameter Difference (%), Speedometer Error. If one input is uncertain, change that value and compare the result again before treating a single estimate as final.

Best-use cases

This page is built around the search intent: compare two different tire sizes, calculate speedometer error from bigger tires, tire diameter calculator. It is most useful when you want to narrow a decision, prepare better questions, or avoid missing a cost, risk, fitment issue, paperwork step, or ownership detail.

Keep the assumptions visible while using the result. If your vehicle, location, driving pattern, quote, loan, insurance policy, or listing situation is unusual, use this as a planning screen and verify the final decision with the relevant document, professional, or local requirement.

Real scenarios

Example calculations

Upsizing for Appearance

Going from 225/65R17 to 245/55R19 on a midsize crossover.

Inputs

Current225/65R17
New245/55R19

Results

Diameter Change+0.74 in
Speedo Error+2.5%

Winter Tire Swap

Downsizing to 205/65R16 winter tires from 225/55R17 summers.

Inputs

Current225/55R17
New205/65R16

Results

Diameter Change-0.23 in
Speedo Error-0.9%

Direct Answer

Changing tire size affects your vehicle's overall diameter, which in turn alters speedometer accuracy and ride height.

Assumptions we made

  • Tires are manufactured exactly to their nominal dimensions

Important limitations

  • Actual dimensions vary slightly between tire brands and models
  • Does not guarantee wheel well clearance

Methodology

How the estimate works

Inputs, outputs, and calculation logic.

Logic

Calculates sidewall height, adds wheel diameter to find overall tire diameter, then calculates percentage differences

Inputs

  • Original Tire Size (e.g., 225/45R17)
  • New Tire Size (e.g., 245/40R18)

Outputs

  • Original Diameter
  • New Diameter
  • Diameter Difference (%)
  • Speedometer Error

FAQ

Frequently asked questions

How do I read tire size numbers?

A tire marked 225/65R17 means 225mm width, 65% aspect ratio (sidewall height as a percent of width), and 17-inch wheel diameter. This tool converts all of that into overall diameter for you.

Does this tool confirm real fitment?

No. It compares dimensions and estimates how the size change affects diameter and speedometer behavior. It does not guarantee clearance or safe compatibility on a specific vehicle.

How much speedometer error is acceptable?

Most drivers and mechanics consider anything under 3% acceptable. Beyond that, your actual speed diverges enough from the dash reading to affect safety and could cause inspection issues in some states.

Will bigger tires hurt my fuel economy?

Larger, heavier tires increase rolling resistance and rotational mass, which can reduce MPG by 1-3%. The effect depends on the size increase and your driving conditions.

What should I compare first: width or diameter?

Overall diameter is usually the first number to check because it affects speedometer behavior and ride height. Width and sidewall still matter, but they do not tell the whole story on their own.

Can I mix tire sizes on the same axle?

No. Tires on the same axle must always be the same size. Mismatched tires cause uneven handling and can damage differentials, especially on AWD vehicles.

Related tools

Continue with the next estimate

Wheel Offset Visualizer

Calculate inner clearance and outer poke changes when swapping to a new wheel width and offset.

Ride Height Change Visualizer

Visually see how changing your car's suspension height by lifting or lowering will affect ground clearance.

Tire Sidewall Visualizer

Calculate and visually preview the sidewall thickness of a tire to see how much rubber you have between the wheel and the road.