Just wondering if there is a big difference between those two diff gears? would there be a big improvement in how hard the car goes if i went from 3.45's to 3.7's? and whats 3.7's like at 100kmph is it starting to rev a bit more?

3.7's are pretty good, I used to run them in my EL. I do a bit of highway work so realistically couldn't go higher than 3.7's with the auto. In saying that I currently have 3.45 in the a*** of my AU and while it is noticably slower it is better for my economy (and currently doing 1000km/week, so need that economy lol).

cheers mate any idea of how hard it was revving at 100km with 3.7's? this is for a manual as well is there much difference between that and the auto maybe just a few 100rpm or something?

Hi i have 3.7's in my EF. It has been a while since I drove the car but I am pretty sure that it sat around 2400RPM at 100km/h. There was a noticable differance in the accelleration of the car, but in saying that I have now bought a 3.9 diff for it because I think that would be a better track/drag ratio for the manual. I don't think it is working to hard on the highway, couldn't tell you if it is getting good fuel ecconamy because I don't care about fuel ecconamy, and the car was modifyed. I could get close to the 550km out of a tank though, if i stayed on the highway and cruised. Definately helped down the 1/4 though I was running the same times as with the 3.45's with 12BHP less. Hope this helps you out

yeah that paints a picture what youve just said, although i dont think i could do 3.9's but for now ill use the 3.45 then look around for a set of 3.7's i think, at least that way can always swap it back if doing a bathurst trip or something