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Automotive

Tire Size Calculator

Compare two tire sizes by overall diameter and percentage difference.

Your measurementsEvery assumption editable01
Size comparisonCalculated live02

Result

+0.19% diameter

Within the usual ±3% tolerance

Current overall diameter225/60R16
26.63 in
New overall diameter245/45R18
26.68 in
Difference
+0.05 in
As a percentage
+0.19%
Current circumference
83.66 in
New circumference
83.82 in
Verdict
Acceptable

Three per cent is the usual working tolerance. Beyond it the speedometer reads meaningfully wrong, ABS and traction control get wheel-speed data they were not calibrated for, and on some vehicles the transmission shift points move.

Overall diameter is what matters, not the section width. A wider tyre on a smaller sidewall can come out the same height, which is exactly what a plus-size fitment is designed to do.

Clearance is a separate question this cannot answer. A tyre can be the right diameter and still foul the strut, the liner or the suspension arm at full lock or full compression.

Enter your current size and the one you are considering. Overall diameter is what actually matters — it sets speedometer accuracy, effective gearing and whether the wheel clears the arch. Three per cent is the working tolerance.

Why use this tool?What it does differently03

Why use this tool?

Diameter, not width

Section width and aspect ratio are only inputs. Overall diameter is the number that drives every consequence.

Tolerance verdict

States plainly whether the change falls inside the usual ±3% window.

Circumference too

Both circumferences shown, which is what determines revolutions per mile.

Honest about clearance

A correct diameter can still foul the strut or liner. The result says so rather than implying a fit.

How this worksThe method04

How this tire size calculator works

Sidewall height is section width in millimetres multiplied by the aspect ratio percentage, converted to inches. Overall diameter is the wheel diameter plus twice that sidewall, since the tyre sits on both sides of the rim.

The percentage difference between old and new diameters is what governs. Within about three per cent the speedometer stays acceptably accurate and wheel-speed data remains close to what ABS and traction control were calibrated against.

This is the entire arithmetic behind plus-sizing: a wider tyre on a shorter sidewall mounted on a larger wheel can produce almost exactly the same overall diameter, which is why 225/60R16 and 245/45R18 are near-equivalents.

How to use itStep by step05

How to use it

  1. Step 1: Read your current size

    Moulded into the sidewall as width/ratio R diameter — for example 225/60R16.

  2. Step 2: Enter the proposed size

    The three numbers of the tyre you are considering.

  3. Step 3: Check the percentage

    Stay within about ±3% unless you are also recalibrating and re-gearing.

  4. Step 4: Check clearance separately

    Diameter is necessary but not sufficient. Width and offset decide whether it actually fits.

Example usageWorked figures06

Example usage

A plus-size fitment
225/60R16 measures 26.63 in. 245/45R18 measures 26.68 in — a 0.19% difference. Larger wheel, shorter sidewall, effectively identical rolling diameter.
Going taller on a truck
265/70R17 is 31.61 in; 285/70R17 is 32.71 in — a 3.49% increase, just outside tolerance. Enough to make the speedometer read noticeably low.
A smaller winter set
Dropping from 245/45R18 to 225/50R17 gives 25.86 in against 26.68 — a 3.09% reduction. Common practice, and right at the edge of the window.
Frequently asked questionsCommon questions07

Frequently asked questions

How much bigger can I go on tire size?

About three per cent in overall diameter. Beyond that the speedometer and odometer read meaningfully wrong and wheel-speed data drifts from what the ABS and traction systems expect.

What does 225/60R16 mean?

225 mm section width, a sidewall 60% of that width tall, on a 16 inch rim. Overall diameter works out at 26.63 inches.

Will a bigger tire fit my car?

Diameter is only part of it. Section width and wheel offset determine whether the tyre clears the strut, the liner and the suspension arms at full lock and full compression — none of which this can check.

Does tire size affect fuel economy?

Yes, in two ways. A taller tyre effectively lowers the numerical gear ratio, and a wider one adds rolling resistance and weight. Both usually cost economy.

What is plus sizing?

Fitting a larger wheel with a proportionally shorter sidewall to keep overall diameter the same. It sharpens steering response at the cost of ride comfort.

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