Highest ratio/lowest number I've seen is 3.57 but I'm looking for one around 3.00. Is there such a beast?
Done some poking around and found the following: Apparently the long ratio here is from the Z33 Auto. While not 3.0, it's in the right direction. Can't verify the info, found here: http://forums.hybridz.org/topic/38251-complete-differential-list-need-help/ and here: http://www.eatsleepboost.lt/tech-info/nissan-tech-info/nissan-r200-diff-ratios/
3.36 is close but no cigar, I'm thinking of experimenting with a much lighter and more efficient three speed auto but they don't have an overdrive. More race car madness. A possible alternative is s diff out of another car, say a Commodore.