Fitting a different tire size changes how fast you really go for a given speedometer reading. Enter your original and new tire diameters and the indicated speed to see your true speed and the error.
Larger tires make the speedometer read low (you go faster than shown); smaller tires read high. Get tire diameter from the Tire Diameter calculator.
How it works
Road speed scales with tire diameter, so actual speed = indicated speed × (new diameter / original diameter). The percentage error is simply how much the new diameter differs from the original.
Tire size and speedometer error
A speedometer is calibrated for the original tire’s diameter, so fitting a different size makes the reading wrong. A larger tire travels farther per revolution, so you’re actually going faster than the speedometer shows (and the odometer under-counts miles); a smaller tire does the opposite. The error scales with the diameter change — a tire 3% larger means you’re going about 3% faster than indicated.
To find true speed, multiply the indicated speed by the ratio of new tire diameter to original — so 60 mph indicated on tires 3% larger is really about 62 mph. Beyond tickets, the error throws off your odometer, trip fuel-economy math, and speed-based features. After a tire-size change, either recalibrate the speedometer (many vehicles allow it) or keep the correction factor in mind.