From what you are saying, CBY, it appears that you want a back axle with a Lower ratio, if you want to be able to reach peak revs in top gear? This will improve your ability to accelerate in the higher gears at the expense of noise and fuel consumption.
A 4.11:1 differential is available, If the factory don't offer it as an option on the Plus 4 then an aftermarket specialist like Billy Bellinger would be able to provide you with one. I have one in my 170bhp 4/4, and it makes for a very lively drive.
But wouldn't it be easier and a great deal cheaper simply to drive around in 4th gear if you want to hit peak revs?
