To offer a fairly basic answer , and to help you understand a little better , think of gear ratio's this way .
The engine turns one full turn than the tranny turns ( I am assuming you have an automatic tranny ) 3.06 times slower.Than the transfer case turns ( assuming low range is selected ) 2.61 times slower,as the driveshft turns the pinion the pinion turns the ring gear and that will be what ever your ring and pinion ratio is ( lets say it is a 3.73 gear ) . The ring gear will turn the differential and the differential turns the axles slower which turn your tires - the differential turns 3.73 slower than the pinion does.
All the reduction added up together is your crawl ratio.
Gear reduction is turning slower than the crank shaft and gear overdrive is turning faster than the crankshaft , while a one to one ratio is turning at the same speed as the crankshaft.
There are factors to consider such as torque converter slip but I am just trying to be very basic so it will be easier to grasp.
HTH , Tom
I edited this with the words "slower" to be a little clearer with what I am saying - sorry for any confusion . Sometimes the written word is difficult to get from my head to typed words.
