I do have a 3.9 h145 centre (pretty sure its 3.9)
Would this be better than the 4.11?
Diffs are something I know nothing about, thanks for the input guys.

its a grey area for me too, in the race cars we use 3.7 and 3.9. i'd assume the more power you have the smaller ratio you could go?? thats only an assumption tho

Anymore input guys?

Have a look at these two links Levey, they helped me quite a bit. Also look at where you engine begins making power/torque from (ie the cams) and plug in your g-box ratios to make sure you got no "holes" between gear changes
http://www.osella.com.au/gear-ratios.htm
http://www.modified.com/editors/techn ... tical_formulas/index.html

Thanks bones I'll have a go at them (this might take awhile!)

more acceleration (and corresponding lower top speed) usually means a numerically higher diff i.e. 4.6. It won't give your engine more power but it will feel like it as it will rev out quicker.

Only difference would be if you had an engine with cruiseship levels of torque that could push a numerically lower diff but I am picking being a A series that ain't the go here

3.90 is the stock Datsun 1200 ratio
4.11 was stock for Datsun 1200 commercials, giving a lower overall gearing for taking off on hills when carrying a load

Generally speaking, the larger ratios give quicker acceleration -- regardless of how much torque or what size engine you have. So 4.11 is "quicker" than 3.9.

But then for top speed, 4.11 will hit redline before 3.9 will -- so lower ratios are "faster" assuming your engine can push the car. The A12 can do it, and will hit redline with 4.11 before 100mph, whereas 3.9 will actually go faster(100mph). 3.7 probably wouldn't let a stock A12 get to 100mph as there is not enough HP. As you can see the factory picked a good overall ratio of 3.9.

So the rule of thumb is:
1. use a higher ratio. 4.865 is quicker than 3.9. If your goal is the 1/4 mile drags, you only need to go about 70mph for a stock A12. So the 4.865 would be perfect, perhaps.

2. but balance that with your need for top-end speed and/or fuel economy. 3.7 is slower than 3.9, but is more fuel-efficient. For circuit racing, figure what your top speed will be and gear for that maximum at redline.

Ok, that pretty much puts things into perspective, thanks DD and to everyone else aswell.