[Greg Needel]

One of the first things I do when designing a transmission is similar to you I take a look at the ratios I want to achieve and then make a spread sheet of the available gears and their pitch diameters. Using this info I can play with gear sizes to obtain the desired ratios.

When going to CAD the transmission I recommend not doing the teeth features to start and just making cylinders that have an outside dimension of the pitch diameter. This makes the components much easier to model and will accomplish the goal of having proper geometric alignment, even if it doesn't look as pretty.

The other thing you should note about 2 speed transmissions is you need to have enough size on the face of the output gear to have a dog (or any other shifting mechanism) engage the face of the gear. Alot of this has to do with how compact you are trying to make your transmission as it will limit the ranges of your shift. Without doing any calculations I suspect a gearbox that has a 4 to 1 shift will have to be rather large even using 20DP gears just due to the sizes required for the gears so their center to center distances align properly. Typically 2 speed transmissions are more in the range of 2-3 times a shift. So you will have to decide if you need that speed range.

If you need any further help, please continue to ask. We all started where you are. Just don't give up we are here to help.