Oh yes, and if you ever read a how-to on re-gearing, you'd understand why. You can't just bolt new gears in. They have to be extremely meticulously adjusted.
To figure out gears, the easiest way is to pop the cover and look. There should be a stamp on the ring gear with at least the tooth counts, but probably the ratio itself too.
If it's just got the tooth counts, divide the bigger number by the smaller number.
Ex. if the stamp is 41 10, 41 รท 10 = 4.10