I had a 3.9 in the car for Bathurst this time around and thought it was almost perfect. I have a silvia gearbox, so the 0.76 5th gear. We were also pulling 6500ish down conrod (240.3 according to GPS) and I thought the car pulled great up the hill, I was hitting 5th just coming over the rise so it was being used for more than just the run down the hill, indicating good gearing. The gearing also worked very well across the top of the mountain with 2/3/4 all being used. It's interesting that we were getting the same downhill speed but you didn't like going up the hill. My car also weighs 1250 with a half tank and me in it, so it's 2+2 territory.
At home here I run a 4.4 and that I think would be too short every wheer and in 2008 I ran a 3.5 diff (0.83 5th gear with a standard 260 box) which was SHOCKINGLY over geared. The perfect gearbox, personally, I think would be a 4.1. There are a couple corners (Murray, Hell, Esses/Dipper/Forrest Elbow) that are about 300-400revs too low with the 3.9 in 3rd but to fast for 2nd and I think a 4.1 would be exactly on the money/power.
OK sounds like that confirms my thinking then. I too was changing to 5th just at the rise going up mountain straight. So it must just be a lack of power that is costing me in the cutting. (I only have 130HP at the rear wheels) Power is not such an issue down conrod.