A 3.9 diff ratio will drop the revs a tad and 3.7 ratio even better if you can find one. 60A 5 sp with the overdrive is even better and atleast 2 times more expensive than a 4 spd. Or if money is no problem, go the 5 spd and a 3.9 / 3.7. So you have to weigh up costs. 1st you have to buy a 60A 4 speed anyway. That may set you back say $300+ and installing say $150 (roughly not a debate people). A 60A 5spd will be roughly $800-$1000 plus installing it. So you have to ask yourself for double the price is it worth what you want to achieve and drop the rev and save fuel and engine life etc. A diff center if you can find one would set you back probably $200+ and then it has to be installed. It will drop your rev's a tad. So a 60A 4 spd and new diff will set you back probably $750+ VS say 60A 5spd $900-$1100 installed. The 5 spd is a better option for long trips or do both. 5 spd s arent that rare , just expensive !!