Tire Size Calculator — Decode Tire Numbers and Compare Sizes

The numbers on the side of your tire — 225/55R17, for example — contain more information than most drivers realize. Width, aspect ratio, construction type, rim diameter, load rating, speed rating. The tire size calculator on CalcHub decodes the full specification and lets you compare two tire sizes side by side to see how switching affects your ride height, speedometer reading, and clearance.

Decoding the Tire Size Code

Using 225/55R17 as an example:

Code Segment	Value	Meaning
225	Width	Tread width in millimeters
55	Aspect ratio	Sidewall height = 55% of tread width
R	Construction	Radial (almost universal today)
17	Rim diameter	Wheel diameter in inches

So: sidewall height = 225 × 0.55 = 123.75 mm Total tire diameter = (123.75 × 2) + (17 × 25.4) = 247.5 + 431.8 = 679.3 mm

Comparing Two Tire Sizes

When you switch tire sizes — upsizing to wider tires, or changing aspect ratio — several things change:

Example: 225/55R17 vs. 235/45R18

Measurement	225/55R17	235/45R18	Difference
Overall diameter	679.3 mm	668.9 mm	−10.4 mm
Sidewall height	123.8 mm	105.8 mm	−18 mm
Circumference	2,133.7 mm	2,100.8 mm	−32.9 mm
Speedometer at 60 mph	60.0 mph	59.1 mph	−0.9 mph

A smaller diameter means your speedometer reads slightly high — you're going slightly slower than it says. A larger diameter means your odometer undercounts miles.

What Changes When You Upsize

Going to wider, lower-profile tires (common for aftermarket wheels) affects:

• Handling: wider contact patch can improve cornering grip
• Ride quality: shorter sidewalls absorb less road shock — harsher ride
• Clearance: taller tires can rub on fenders or suspension components
• Fuel economy: wider tires generally increase rolling resistance slightly

Staying within ±3% of your stock tire's overall diameter is the general guideline for not affecting speedometer accuracy significantly. Always verify tire fitment with a professional or your vehicle manufacturer's specs. Incorrect tire sizes can affect speedometer accuracy, ABS function, and clearance. Do not use this as a sole guide for purchasing decisions.

Does tire width affect speedometer accuracy?

The speedometer calibration depends on tire circumference (rolling diameter). A taller or shorter tire changes how many rotations per mile your wheels complete. A 3% larger circumference means your odometer undercounts by 3% — and your speedometer reads 3% slower than actual speed.

What's the 3% rule for tire sizing?

Keeping your new tires within ±3% of your stock tire's overall diameter minimizes speedometer error and keeps you within acceptable range for most vehicle systems. Beyond that, some vehicles require speedometer recalibration, and ABS/traction control systems may behave differently.

Can I put wider tires on my stock wheels?

Only within a range. Each wheel rim has a minimum and maximum tire width it can properly mount. A 7-inch wide rim, for example, can typically mount tires from 195mm to 235mm wide. Going outside the approved range creates safety risks during mounting and operation.

Related Calculators

• Car Depreciation Calculator — full vehicle value analysis
• Fuel Cost Calculator — tire choice affects fuel economy
• Gear Speed Calculator — how tire diameter affects gear ratios