[427sharpe]

The '90 gearbox should have the 8 tooth drive gear (inside the trans), and you need a 2.33 ratio; if your speedo is pegged to 1000 RPM @ 60, which is pretty much standard. So look for a 19 tooth gear. If yours has the old 7 tooth drive, look for a 17 tooth gear. You can see the internal gear thru the speedo cable hole, the 8 took gear is green, the 7 is yellow. 6 is also available, and is black. It would be a part installed during a rebuild at some point. If you see that one, you need a 14 tooth gear. This combo would be the most accurate for you if you have the trans out and can replace the drive (internal) gear.