Can anyone tell me what the original size tires were from the factory
on the 1987 Civic Si?

If someone is good at math, can you tell me what the speed difference
would be, as indicated on the speedometer, between this stock size and
the currently installed P185-65-R15 at, say, 60mph?

Also, where can one obtain the driven gear (or whatever it is called)
that would correct the speedometer for the above current tire size?

Thanks a bunch.