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Just think about it like this. The usual standard diff ratio on a commodore is 3.08 - 1
Now think of that as first gear on a mountain bike. You have to peddle hard to get it moving right??
Now put the bike in a higher gear, say 4th. Its a lot easier to get it moving yeh? you dont have to work as hard.
Now if you put a higher diff gear in your commodore (correct term is taller and shorter but its easier to understand like this), say 4.11, your car wont have to work as hard to get moving.
The gears go up in these increments; Standard is 3.08 and 3.46, upgrades are 3.73, 3.89 and 4.11. the higher you go the less your car has to work to get moving therefore you will have faster acceleration, however, the higher you go, the less your top speed will be.
Although diff gears wont affect you ET's much, it will make a difference in your 60 footers as it helps you get out of the hole.
If you mostly drive in the city/suburbs i would recommend 4.11, you wont look back. But if you do quite a bit of freeway/highway driving maybe 3.46 or 3.73. However i stll went 4.11 and havent look back.
Why? you ask. Because the higher the diff gear the more you'll be revving at freeway speeds 100kmph. But if you can put up with drone (if your exhaust system has drone), and dont mind paying $5 extra in petrol a week go 4.11, either way you wont regret it.