Tire Size Calculator

Sidewall height is the aspect ratio times the width, converted from millimeters to inches by dividing by 25.4. Overall diameter adds two sidewalls (top and bottom, since the wheel sits between them) to the wheel diameter. Circumference is diameter times pi, and revolutions per mile is 63,360 (the number of inches in a mile) divided by that circumference.

For the default 215/65R15: sidewall height is 65% of 215mm, converted to inches, about 5.50 in. Overall diameter is 15 plus two sidewalls, about 26.00 in. Circumference is pi times 26.00, about 81.7 in. Revolutions per mile works out to roughly 776.

"Plus sizing" means moving to a larger wheel diameter while lowering the aspect ratio to keep the tire's overall diameter close to the original, a common upgrade when buying wider, lower-profile wheels for looks or handling. The same comparison also covers the opposite situation: checking whether a used, mismatched spare or a size swapped in for winter tires is close enough to your factory size to drive on safely. Turn on Compare to a Second Tire and enter both sizes to see exactly how close a candidate size actually lands.

Work through the same sidewall math for the second tire: a 225/60R16 has a sidewall height of 60% of 225mm, converted to inches, about 5.31 in, giving an overall diameter of 16 plus two sidewalls, about 26.63 in. Compared against the first tire's 26.00 in, that's roughly 2.4% larger overall.

A common guideline among tire shops is to keep that difference within roughly 3%. A bigger jump can throw off your speedometer and odometer readings, reduce ground clearance more than expected, and on some modern vehicles, confuse traction control or all-wheel-drive systems that rely on all four wheels reporting a consistent rotational speed.

Your speedometer doesn't measure road speed directly; it counts wheel revolutions and assumes a specific tire diameter to convert that into miles per hour. Fit a taller tire and each revolution covers more real distance than the speedometer assumes, so your actual speed runs higher than the number on the dial. Fit a shorter tire and the reverse happens: the dial reads high while your real speed is lower, which matters at highway speeds where even a small error compounds over a long drive.

The comparison result shows your actual speed when the speedometer reads 60 mph on the new size. For the default comparison, a 2.4% larger tire means the dial reads 60 while you're actually doing about 61.4 mph. If you change tire size permanently, the Speedometer Gear Calculator can work out the gear change needed to correct the reading itself.

A full tire code carries more than width, aspect ratio, and wheel diameter. A letter prefix (P, LT, or ST) marks the vehicle class the tire is built for: P for passenger cars, LT for light trucks built to carry heavier loads, and ST for trailers. LT tires are often built with a higher load range and stiffer sidewalls than a P-metric tire of the same nominal width and aspect ratio, so swapping between the two classes at the same numeric size doesn't necessarily give you the same real dimensions or ride quality. After the wheel diameter, a load index number states the maximum weight the tire can carry, and a speed rating letter states the top speed it's rated for at that load. A separate DOT code, usually the last four digits, gives the week and year the tire was manufactured, useful for checking tire age regardless of tread wear. This calculator focuses on the size geometry; load index, speed rating, vehicle class, and manufacture date don't change a tire's physical dimensions, so they're outside its scope.

This calculator is part of a small family of automotive wheel and drivetrain tools:

• Speedometer Gear Calculator, works out the driven gear change needed to correct a speedometer after a tire size change
• Wheel Offset Calculator, checks fender and suspension clearance when switching wheel width and offset
• RPM to MPH Calculator, converts engine RPM to road speed using tire diameter and axle ratio