Reference library

Tire size guides

Practical, jargon-free guides on reading sidewalls, checking fitment, calculating plus sizing, and understanding speedometer error. Calculations describe nominal geometry; installation still needs verification.

Guide safety note: these articles explain nominal tire geometry. They do not replace a vehicle-specific fitment check by a tire shop.

Tire basics

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.

Read guide

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.

Read guide

Fitment checks

Will this tire size fit my car? What the math can tell you

What nominal dimensions tell you, and which tire, wheel, and vehicle details still need checking.

Read guide

How a different tire size changes your speedometer

Estimate the speed and distance effect of a diameter change, with clear assumptions.

Read guide

Sizing & upgrades

Plus sizing explained: larger rims and nominal diameter

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

Read guide

Buying tires

Wider tires: pros, cons, and the real cost

What changes with width, and what depends on the actual tire model and wheel setup.

Read guide

TreadSize calculates dimensions from nominal size markings. Confirm the actual tire, wheel, and vehicle specifications before installation.