Tire size guides
Practical, jargon-free guides on everything from reading a sidewall to calculating a plus-size upgrade. All math is exact — no vague rules of thumb.
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.
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.
Fitment & safety
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.
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.
Sizing & upgrades
Buying tires
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.