The original size is 175/70 13 for a tire diameter of 22.65" and a
circumference of 71.14"
Your size is 185/65 15 for a diameter of 24.47" and a circumference of
76.87".

The difference is 8%, your speedometer will show 8% lower than it would
with stock wheels. If it showed 60 with stock wheels it would show 55.8
with the wheels you have. I don't know how accurate the speedometer is
in and of itself, but it is not unusual for speedometers to show a
percent or two too high a reading with stock tires.

Do a google search for "speedometer calibration" to find places that can
calibrate your speedometer.

Tina Venne wrote:
>
> 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?