Determining spur gear diameter given distance between shafts

[Varthrix]

Hello guys, first of all excuse me if I make mistakes, but English is not my native language.
So, I am creating a 4 speed gearbox, I know the ratios of the gears I will be creating, I know the distance between the primary shaft ( the one coming for the engine`s flywheel ) and the secondary shaft ( the one that turns the differential ) and I want to be able to calculate the diameter of the spur gears. Can you give me a calculator or a formula on how to do this?

 

[Baluncore]

If you know the ratio of a gear pair then you know their pitch circle radii because the gears must share the space between the shafts in proportion to their ratio.

You must then work out the closest integer ratio to identify the number of teeth on each gear. Those teeth must be big enough to carry the load and small enough to not interfere or undercut.

Try this;
http://www.qtcgears.com/QTC_Redo/Product-Technical-Information/Worm-Gears-Technical-Information.htm

Get the technical reference .pdf at the bottom of this page;
http://www.khkgears.co.jp/en/gear_technology/guide_info.html