Hot Rods tach drive ratio adapter question

I plan to use a late 50s Corvette tach drive generator and SW 5000 tach... the generator pulley is 3" diameter and the crank pulley is 6 1/2" diameter (324 Olds)... what ratio adapter will I need?... Thanks for the help...

 

get it put together without an adapter, and see how far off it is, and go from there?

 

Talk to rusty

 

Try this, https://www.blocklayer.com/pulley-belteng

 

If you don’t have a way to check the RPMs a different way (wire up an electric tach temporarily), you will need to find the diameter of the original Vette crank pulley. Then you could find the difference in circumference between the old and the current pulleys. That would be needed to see what percentage you are off.