Hi.

You wrote, "...You wrote that your speedometer gives you 67 mph when you're really going 63 mph...." Are you sure you didn't mean the opposite? You're riding on bigger tires, so you should be going faster than your speedometer, rather than slower, because all your speedometer really knows is how fast your tires are turning, not how far they go with each revolution.

Your original tires' (OEM size) diameter:
(185 * 0.70 * 2) + (14 * 25.4) = 614.6 mm

[the 0.70 factor is the aspect ratio = "70 series" tire.
the * 2, or times 2, factor is because there are two sidewalls for the overall diameter.
the 25.4 factor is the number of mm/inch for the wheel, to make the overall diameter in mm units.]

Your new size's diameter:
(195 * 0.75 * 2) + (15 * 25.4) = 673.5 mm

the new tire is 673.5/614.6 = 1.096, or 9.6% larger (because of both the increased aspect ratio (.75 vs .70) giving a taller sidewall, and also larger wheel (15 vs 14).

BTW, this isn't "slightly larger". This is essentially 10% larger, which is a lot: you should be going 10% faster, and 10% farther, than your speedo/odometer indicates.

If we assume that you meant to write the reverse, i.e. that 'you're really going 67 when your speedometer reads 63 mph', this would be 6.3% faster, which isn't quite as great a discrepancy as would be predicted. I would have predicted that you're really going 69 mph; how serious this is depends on the leeway that cops in your area give away :-) .
Did you time the seconds on a highway's quarter mile painted lines, or roadside markers? Or use a GPS?
Probably, your speedometer isn't perfectly accurate (they never are, except on cops' "certified" speedometers).