TireCalc
Tire size comparison, speedometer correction, wheel offset, bolt patterns, and load ratings. The numbers behind your next tire purchase.
Tire Size Comparison
Compare two tire sizes side-by-side. Diameter, width, sidewall, and speedometer difference.
Speedometer Correction
Calculate actual speed when running non-stock tire sizes.
Wheel Offset Calculator
Calculate wheel offset and backspacing. Check if new wheels will fit.
Bolt Pattern Reference
Look up common bolt patterns and PCD by vehicle make.
Load Index & Speed Rating
Look up tire load capacity and speed rating from the sidewall code.
About TireCalc
TireCalc provides free tire and wheel calculators for car enthusiasts, mechanics, and everyday drivers. Compare tire sizes, check speedometer accuracy, calculate wheel offset, verify bolt patterns, and understand load ratings.
All tools run in your browser. Share results via URL — send a tire comparison to your mechanic or save it for the tire shop.
Tire size notation: a tire labeled 225/55R17 93H decodes as: 225 = tread width in millimeters; 55 = aspect ratio (sidewall height as % of tread width, so 225 × 0.55 = 123.75mm sidewall); R = radial construction; 17 = wheel diameter in inches; 93 = load index; H = speed rating. Total tire diameter = (sidewall height × 2) + wheel diameter converted to mm. (123.75 × 2) + (17 × 25.4) = 247.5 + 431.8 = 679.3mm ≈ 26.7 inches.
Speedometer error from tire size changes: speedometer calibration is based on the factory tire's rolling circumference. If you install tires with a larger overall diameter, actual speed is higher than the speedometer reads. Speedometer error % = (new circumference − old circumference) / old circumference × 100. A 1% larger diameter causes approximately 1% speedometer reading error. Most manufacturers allow ±3% before speedometer recalibration is needed.
Wheel offset (ET): offset is the distance (mm) from the wheel's mounting face to its centerline. Positive offset means the mounting face is toward the front of the wheel (OEM on most FWD vehicles). Negative offset pushes the wheel outward. Backspacing = (wheel width / 2) + offset (converting all to inches). Changing from OEM offset can affect suspension geometry, bearing load, and cause rubbing — always verify clearances with the actual wheel installed before driving.
Bolt pattern measurement: bolt pattern is expressed as number of bolts × bolt circle diameter (e.g., 5×114.3). The bolt circle diameter is the circle that passes through the center of all wheel studs. For a 5-lug wheel, BCD = stud-to-stud distance (measuring across, skipping one) / 0.9511. Precise BCD requires measuring with calipers or using a bolt pattern gauge — estimating from a tape measure has significant error on tight spacing.