Tire Size
Calculator & Converter

Compare two tire sizes, check sidewall, overall diameter, circumference and speedometer error before you buy or upgrade.

All metric calculations Speedometer error check Safe 3% upgrade rule

Compare tire sizes

Original / Tire 1

Width (mm)

Aspect (%)

Rim (in)

New / Tire 2 (optional)

Width (mm)

Aspect (%)

Rim (in)

Diameter difference

+7.5 mm · +0.46%

✓ Within the safe 3% upgrade rule

Speedometer error

Reading 3.0 km/h low

At a shown 100 km/h, actual speed ≈ 100.5 km/h

Rev / mile

−4 revs/mile

Learn

How to read a tire size

A tire size like 225/45 R17 contains three numbers:

225
Section width
Tire width in millimeters, from sidewall to sidewall.
45
Aspect ratio
Sidewall height as a percentage of width: 45% of 225 mm = 101 mm.
R
Construction
Radial — the standard construction for modern tires.
17
Rim diameter
Wheel diameter in inches the tire fits on.

Upgrading

The safe 3% rule

When changing tire size, keep the overall diameter within 3% of the original. Beyond that, the speedometer becomes inaccurate and ABS/traction control can misbehave.

Going wider (e.g. 225 → 235 mm) adds grip but increases rolling resistance and fuel use. Going to a lower aspect ratio (e.g. 45 → 40) firms the ride and sharpens handling.

Note: this calculator gives metric geometry, not fitment. Always verify wheel offset, brake clearance and arch clearance on your specific car, and check your region's legal tire-size rules.

FAQ

Tire size FAQ

How do I read a tire size like 225/45 R17?

225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 17 is the rim diameter in inches. So 225/45R17 has a 225 mm wide tire, 101.25 mm sidewall, on a 17-inch wheel.

Can I put larger tires on my car?

Usually yes, within limits. Keep the overall diameter within 3% of the original to avoid speedometer errors, rubbing and ABS problems. Use the comparison tool to check the difference before buying.

How much can tire size affect the speedometer?

If the new tire is larger than original, your speedometer reads lower than your actual speed (you are going faster than shown). A 3% diameter change means about 3% speed error — enough to matter.

What happens if I change tire size without adjusting?

The speedometer and odometer become inaccurate, ABS and traction control may misbehave, and the wrong size can rub or fail inspection. Always match overall diameter as closely as possible.

What does the aspect ratio mean?

It is the sidewall height as a percentage of the section width. A 225/45 tire has a sidewall of 45% of 225 mm = 101.25 mm. Lower numbers = shorter sidewall = sportier, firmer ride.

What is a common tire size upgrade?

Going from 225/45R17 to 235/45R17 (wider, +10 mm width) is common and stays within the safe 3% rule. Always verify offset and clearance on your specific car.

How do I convert tire diameter to inches?

Divide the overall diameter in millimeters by 25.4. For a 225/45R17, diameter is 634.3 mm, which is about 24.97 inches.

Does tire size affect fuel consumption?

Yes. Wider and larger tires add rolling resistance and weight, typically increasing fuel consumption slightly. Smaller/narrower tires generally save fuel.

EV vs petrol →

Compare running costs — bigger tires use more energy.

OBD-II codes →

Decode check engine codes for free.

Crypto converter →

Convert 20+ cryptocurrencies to 30 currencies.

More auto tools