Help with speedometer ?

[Mopsdrops]

Hi,

Can some one please help me with this one ?
I have some pocket bikes and I like to put a speedometer on it, but I don't find any speedometer especially for pocket bikes, does anybody know what kind of speedometer I can use on a Pocket Bike ? Is a speedo of a bicycle also good ? or is the diameter of my rim to small ? it's 6,5" .

Thanks