How to Read Tire Size: Width, Aspect Ratio and Rim Diameter
Decode 225/45R17, understand tire width and sidewall height, and separate the size marking from load ratings and actual measured dimensions.
THE REFERENCE DESK
Tire and wheel guides
Understand the measurements before comparing a setup. These guides explain tire sizes, wheel position and fitment changes with worked examples and links to the right calculator.
Start with the question you need to answer. Tire size markings describe nominal dimensions; wheel offset and backspacing use different references. Similar numbers alone do not establish vehicle compatibility.
Tire sizes and speed
Tire Size Conversion: Metric Sizes, Inches and Flotation Markings
Convert metric tire dimensions to inches, decode 33x12.50R15 flotation sizes, and understand why a dimensional match is not an approved replacement.
Plus Sizing Tires: Compare a Larger Wheel Without Guessing Fitment
Understand plus-one and plus-two tire sizing with a worked 17-to-18-inch example, diameter comparison and a practical replacement checklist.
Tire Size and Speedometer Error: Calculate the Direction and Amount
See how larger or smaller tires affect estimated road speed, with mph and km/h examples and clear limits for nominal tire-size calculations.
Tire Circumference and Revolutions per Mile: Nominal vs Rolling Data
Calculate theoretical tire circumference and revs per mile or kilometer, then understand why manufacturer rolling specifications can give a different number.
Wheel offset and backspacing
Wheel Offset Explained: Positive, Negative, and ET Numbers
Understand positive and negative wheel offset, read ET markings, and calculate how an offset change moves a wheel toward the suspension or fender.
Wheel Width and Offset: Calculate Inner Clearance and Outer Position
Compare wider wheels and offset changes with a worked 18x8 ET35 to 18x8.5 ET42 example. Separate nominal wheel position from tire and rim-lip clearance.
Offset vs Backspacing: How to Measure and Convert Them
Learn the difference between offset and backspacing, measure the correct wheel surfaces, and convert using explicit overall-width or inner-flange geometry.
Wheel Spacers and Effective Offset: Understand the Geometry
Calculate effective wheel offset with a spacer, understand per-side thickness versus axle track change, and identify the fitment questions geometry cannot answer.
Checking a complete setup
Wheel and Tire Fitment Checklist: What to Check Before a Change
Plan a wheel and tire change with a practical checklist for dimensions, offset, tire ratings, measured clearance, brakes, and manufacturer confirmation.