You don't need to use paspeedo.com, all you need is a calculator, or a piece of paper with old time basic math skills.
One of the nice things about the tire sizes of the last twenty years or so is that you can easily calculate the diameter from the size.
First Number/25.4 X second number as a decimal X 2 + dia.
example: 195/65 - 15
195/25.4 x .65 X 2 + 15 = 24.98" diameter
This is the calculated tire diameter, if you need the circumference, you can simply multiply times pi to get the calculated circumference. The calculated circumference will not be the same as the rolling circumference but close enough for most purposes.
If you wish to compare one tire size to another to determine the speedometer error percentage, simply calculate the two sizes and divide one by the other to determine the percentage difference. The diameter is directly proportional to the circumference, so there's no need to calculate the circumference to derive the percentage difference.
Have a great day,