Speedometer Gear for Km?

think you'd be better off either getting a metric speedometer, or get a speedometer calibration gear box that attaches to the cable

I agree the calibration box would be the best bet.

OR

You just memorize the key mph that are kph like 62 is 100 etc.

By the way the most accurate way to get the correct gear is what the speedo says and what a GPS says and the number of teeth on your current gear. Speedo GPS g numbers gets you a ratio, that's the ratio to apply to current teeth to determine the new teeth. Fewer teeth makes speedo read faster