Hi Spyke,
Check out the link and spreadsheet that T20racerman and MG recently uploaded. These will answer your question and many more.
Go to the "Clutch, Primary, Gearbox" section and check out the thread entitled "Vaguely useful spreadsheet to calculate speed from different sprocket set ups". It's only recent so it will be close to the top of the list.
When you open it up click on the link for the Sprocket Set up information and/or scroll down the thread to find the Gear Ratio Spreadsheet.
These are both excellently presented and easy to understand.
Beezageezauk.