Tire size calculator
Enter your current size and the one you're considering. Metric (265/70R17) and inch (33x12.50R17) sizes both work.
How to read a tire size
265/70R17: 265 is the section width in millimetres, 70 is the aspect ratio (sidewall height as a percentage of the width), R means radial, 17 is the rim diameter in inches. A leading P means passenger, LT light truck, ST trailer. Inch sizes like 33x12.50R17 give the overall diameter and width directly.
The 3% rule: a replacement whose overall diameter is within ±3% of the original keeps the speedometer, odometer, ABS and gearing close enough. Taller tires read slower than you're going; shorter tires read faster.