i have worked it out once before, here's my rough recolection. its probably wrong but hopefully you'll see my mistake and be able to work it out.

RPM x {final drive of gearbox(1/.82)}
divide by diff ratio (4.625)

this gives you revolutions of your tyre per minute.

then times by the circumference of your tyre (you'll need to measure this)

this will give you a meters per minute figure. then times by 60 and this gives you metres per hour. then divide by 1000 and you'll get kilometers per hour

any more input people?? if i guestimate your wheel size i got 254kph but i'm very doubtful a 4.625 would get that high...

by the way, are those road tyres????