Follow this video on how to determine your gear ratio if it isn't listed anywhere. This is a car yes, but it's the same concept for a Bus. Just heavier.
It does require you lift the bus up in the air with a jack or two so be careful in doing so.
To safely do this, get 4, 4x4 blocks of wood and put them under both sides of the front tires to prevent rolling. This is mandatory because you'll need to put the bus in Neutral and it has potential to roll, so also ensure the ground is as flat and level as possible.
(I own 2x 6 ton jacks with keys), and a bottle jack to get it up on one side, use the bottle jack on one rear side, put 1 jack under it, release the bottle jack, and then do the other side, put the 2nd jack under it, release the bottle jack, You only need to barely get the two rear wheels up off of the ground.
Once you've accomplished this safely, follow the turning guide here, and count wheel revolutions and differential revolutions to obtain your gear ratio information.