Try these calculators to work out what will either get you there or close:

http://www.tchc48-78.com/speedocalc.html

http://www.chevymania.com/tech/speedo.htm

Rares do a heap of drive gears that may get you where you work out you need to be:

http://www.rarespares.net.au/rarespa...n=speedo+drive

Regards,

Dave