GarageCalc 
How to use the chart
Find your wheel diameter, then your size. The number next to each size is its overall diameter in inches, which is the figure that matters when you change sizes: keep the new tire within 3 percent of it and the speedometer, ABS and gearing stay effectively stock. Widths run from 145 to 325 mm and aspect ratios from 30 to 85 percent.
A size that is not in the chart still works in the tire size calculator, which accepts any width, aspect ratio and wheel diameter combination.
Frequently asked questions
How do I find my tire size?
Read the sidewall or the placard inside the driver's door jamb. It looks like 225/45R17: width in millimeters, sidewall height as a percentage of width, R for radial, and the wheel diameter in inches. Pick that size in the chart to see its dimensions and equivalents.
What does the chart show for each size?
Overall diameter in inches and millimeters, section width, sidewall height, circumference and revolutions per mile, all computed from the size code, plus every other common size within 3 percent of the same diameter. Sizes run from 155/70R12 (20.5 in tall) to 325/60R20 (35.4 in).
Are two tires with the same diameter interchangeable?
Only if the wheel diameter matches and the rim width suits the tire width. Within 3 percent of diameter the speedometer and gearing stay effectively stock; past that, clearance and calibration become real problems. Each size page ranks the alternatives by that difference.