Tire Size Calculator
Compare your current tire size against a new one and see the real diameter difference, speedometer error, and revolutions per mile.
Current tire
New tire
Current diameter
26.63"
New diameter
28.95"
Diameter change
+8.72%
Your speedometer will read about 8.7% slower than your real speed unless it's recalibrated. Changes over ~3% are commonly flagged as worth correcting.
Current: revolutions/mile
757
New: revolutions/mile
697
Tire sizes use the standard sidewall format -- e.g. a 225/60R16 is 225mm wide, with a sidewall height 60% of that width, on a 16-inch wheel.
Frequently Asked Questions
Why does a bigger tire throw off my speedometer?
Your speedometer is calibrated to your original tire's revolutions per mile. A larger tire covers more real distance per revolution, so at any given wheel speed, you're actually going faster than the speedometer shows -- by roughly the same percentage as the diameter increase. The percentage shown is the diameter-ratio convention tire shops and Tire Rack use; it is an approximation that drifts by several points at very large size changes, so treat a 15%-plus swap as a recalibration job, not a number to trust to the decimal.
How much of a size difference is safe to run?
A common rule of thumb among tire shops and forums is to stay within about 3% of your factory diameter to avoid speedometer/odometer error, ABS or traction-control miscalibration, and clearance issues. Bigger changes are common in off-road/lifted builds, but usually need a speedometer recalibration and sometimes a lift kit for clearance.
Does this replace a real fitment check?
No -- this calculates the math (diameter, circumference, revolutions per mile), not physical clearance against your specific fenders, suspension, or lift height. A real shop can check actual fitment for your vehicle before you buy.
Upgrading your car's whole look, not just its stance?
AutoGlossHub tracks real, verified vehicle wrap, tint, detailing, PPF, and ceramic coating shops -- the same crowd that's usually swapping wheels and tires too.