How a Change in Tire Size Affects Speedometer Accuracy
When you change out your tires and wheels for a larger wheel, you always want to respect the overall diameter (OD) of the original tire. A lower profile tire must be placed on a larger wheel to keep the OD close to the same as the tire/wheel combination being replaced. You certainly never more than a 5% difference in OD to the original tire. Too large a variation could impact the gearing and correct operation of the vehicle. Also, any change in the tire OD will give you a false reading of speed and distance on your odometer.
When your vehicle was new, the speedometer was calibrated by the factory according to the exact size of the intended tires. If you change to a taller tire, the circumference of the tire will also be greater. What this means is one rotation of the tire will take you further on your new tires than on your old tires. If the speedometer was never re-calibrated with the new tires, it will register a slower speed than you are travelling. For example, if your tire was 3% taller (still within approved guidelines for plus fitments) the gearing and operation of the vehicle would be fine, however your speedometer would show 60 mph, when in fact you are traveling 63.3 mph.
An easy way to measure your speedometer accuracy it to run a road test. Freeways have mile markers that indicate the length of each mile you are traveling. The safest and most accurate way to execute this test is to have a passenger in the car with a stopwatch. Set your cruise control at 60 mph, and start the stopwatch when you pass the mile marker. It should take you 60 seconds to pass the next mile marker. Repeat the test three or four times to be sure, and average the resulting times. If your average time is off by 3 seconds or more, your speedometer is likely in need of re-calibration. If you have changed out the tire and wheel package, the OD and resulting change in revolutions per mile is likely the cause.