the higher the first number, the longer the ratio-n
with your 3.92:1 the prop will turn 3.92 times in order to turn the crownwheel once
with my 3.14:1 diff, the prop only needs to turn 3.14 times in order to turn the crownwheel once

there are lots of people who say you would need more power to turn the longer diff, but, as witnessed in the fsoc rr day in ff, one i had the lowest powered twinky on the day but, at ford fair last year, i absolutly blitzed the guys with more powerfull cars around the track (several times in fact because i kept going off at one particular corner ) because it's all a question of how you make the power and how you use it