Comment: Tyre Size Calculator

Why is the measured tyre size (approximately) 5% lower than the online calculator?
I measured the driven tyre of my car, and also a family members car. My tyres is a Good Year, and the other car is using the Yokohama brand. My tyres measured 95.2% lower than the online calculated diametre tyre size, and the other car's tyre was 94.2%. There is appropriate air pressure in tyre, and i checked the wooden ruler with a tape measure to see if there were any innacuracies. There were no anomalies to explain the roughly 5% difference.
Does the tyre really expand that much due to centrifugal force when driving? Somehow i am not convinced the tyre would contort that much. Does anyone know how much the tyre may expand in actual use?
Perhaps it is just the bean counters (ie Accountants from the tyre manufacturers) "robbing" the consumer by giving us less tyre?
I need to know this, because i am calculating gear ratios, top speed, and torque at the wheels for an electric car conversion. Depending on which car and motor combination i use, knowledge of the tyre size could be near critical to my calculations.
Thanks
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