Tire Size Calculator – Compare Stock vs New

How the calculations work

Tire sizes are given as Width/Aspect R Rim, e.g., 225/45R17.

• Sidewall Height (mm) = Width × (Aspect ÷ 100)
• Rim Diameter (mm) = Rim(in) × 25.4
• Overall Diameter (mm) = 2 × Sidewall + Rim(mm)
• Circumference = π × Diameter
• Revs per mile = 63,360 ÷ Circumference(in)
• Revs per km = 1,000,000 ÷ Circumference(mm)
• Speedometer: Actual = Indicated × (New Diameter ÷ Stock Diameter)
• Ride height change = (New Diameter − Stock Diameter) ÷ 2

Note: This calculator compares tire sizes only. Wheel width, offset, brake and fender clearances, and load/speed ratings are not assessed.

Tire Sizing 101: What the Numbers Mean and Why They Matter

Modern tire codes look cryptic at first glance—something like 225/45R17—but each part describes a dimension that affects how your car accelerates, brakes, corners, and reports speed. The first number is the section width in millimetres (225 mm here). The second is the aspect ratio, a percentage of the width (45% of 225 mm) that determines sidewall height. The letter R indicates radial construction, and the last number is the rim diameter in inches. From these, you can derive overall diameter, circumference, and revolutions per distance—exactly what this calculator shows.

Key formulas at a glance

• Sidewall height = width × (aspect ÷ 100)
• Rim diameter (mm) = rim(in) × 25.4
• Overall diameter = (2 × sidewall) + rim(mm)
• Circumference = π × diameter
• Revs per mile = 63,360 ÷ circumference(in)
• Revs per km = 1,000,000 ÷ circumference(mm)
• Actual speed = indicated × (new diameter ÷ stock diameter)

How close is “close enough”?

A common rule of thumb is to keep overall diameter within roughly ±3% of the original size to preserve speedometer accuracy, gearing feel, and fender/brake clearance. Larger diameters reduce revs per mile (taller gearing), often showing a lower indicated speed than actual. Smaller diameters do the reverse. Our results call out diameter deltas and speedometer error so you can judge suitability quickly.

Real-world considerations beyond diameter

• Wheel width & offset: Even if diameter matches, an overly wide tire can rub struts, liners, or fenders—offset and wheel width determine stance and clearance.
• Load index & speed rating: Match or exceed OEM specs to maintain safety, handling, and insurance compliance.
• TPMS & calibration: Significant size changes can affect ABS/TC logic and TPMS thresholds; some vehicles allow speedometer/odometer recalibration.
• Season & tread type: Winter tires often run narrower for traction and slush evacuation; ultra-high-performance summer tires may square up wider on the same wheel.
• Unsprung mass & inertia: Bigger wheels/tires can add weight, affecting ride and acceleration; lighter assemblies improve response.
• Gear ratio feel: Fewer revs per mile = taller gearing (lower engine rpm at a given road speed), which can slightly soften launch but reduce cruise rpm.

Worked example

Compare 225/45R17 to 235/40R18. The 18-inch option typically has a shorter sidewall to offset the larger rim, keeping the overall diameter similar. If the calculator shows a diameter change near zero, speedometer and gearing feel remain close to stock; if it shows ~2–3% larger, expect a small actual-speed increase versus indicated and slightly taller gearing.

Tips for choosing an alternative size

• Start from OEM size and target ±3% diameter or less.
• Check wheel manufacturer fitment guides for width/offset compatibility.
• Verify load index/speed rating; do not down-spec for cost.
• Inspect clearances at full lock, full bump (suspension compressed), and with chains if applicable.

Disclaimer: This tool estimates geometry only. It does not guarantee fitment or legal compliance. Always confirm clearances on-vehicle and follow manufacturer specifications.

Frequently Asked Questions

Which formats can I enter?

Try 225/45R17, 225/45-17, or 225 45 17. Width and aspect are metric; rim is inches.

What’s a safe diameter change?

Many guides suggest keeping overall diameter within ~3% of stock for speedometer accuracy and clearance. This is not a guarantee—always check clearances.

Why do revs per mile matter?

They relate to gearing and speedometer calibration. Fewer revs/mile typically means a larger diameter tire.

Is my data private?

Yes. Everything runs locally in your browser.