TreadSize

Fitment library

Tire size guides

Practical, jargon-free guides on reading sidewalls, checking fitment, calculating plus sizing, and understanding speedometer error. The math is exact; installation still needs verification.

Guide safety note: these articles explain tire math and common tolerances. They do not replace a vehicle-specific fitment check by a tire shop.

Tire basics

Close-up tire tread with a TreadSize sidewall decoding card

How to read a tire size: the complete breakdown

What 215/55R17 actually means — every number and letter decoded, including load index and speed rating.

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Mounted wheel and tire with a TreadSize tire-versus-wheel measurement card

Tire size vs. wheel size — what's the difference?

The rim diameter is one number in the tire size. Here's how the two specs relate — and why both matter.

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Fitment & safety

Tire under a wheel well with a TreadSize fitment clearance checklist

Will this tire size fit my car? (The 3% rule explained)

The one rule that determines if a tire swap is safe — and exactly how to apply it to any two sizes.

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Car speedometer close-up with a TreadSize speedometer-error card

How a different tire size changes your speedometer

Larger tires make your speedo read low; smaller ones make it read high. Here's the exact math.

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Sizing & upgrades

Garage tire stack with a TreadSize plus-sizing path card

Plus sizing explained: +1, +2, and the diameter rule

Go up a rim inch, drop the aspect ratio, keep the diameter. Here's how the math actually works.

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Buying tires

Close-up tire tread with a TreadSize wider-tire tradeoff card

Wider tires: pros, cons, and the real cost

More grip, more rolling resistance, more noise — and yes, they can raise your running costs.

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All tire size math on TreadSize uses standard ETRTO formulas. Guides are reviewed and updated periodically. Always confirm fitment with a qualified tire professional before purchasing.