Not quite the same, but probably the best mod I have had done is changing the diff ratio from 3.97:1 to 4.1:1 makes the car so much more drivable. 30 mph in 4th & 40 in 5th perfectly doable. 70 mph on the tacho = 3000 rpm. About 300 rpm more than standard. Would recommend. Just 1 comment, for some reason the diff does “ring” a bit. Seems the replacement gears are not that well manufactured. For those interested the “ring” starts to show above 50mph & varies a little depending rpm. The “ring” unfortunately occurs only when cruising. Accelerate it stops deceleration it stops. Having said that it is not bad, can’t be heard roof down. (2011 4/4, 1600 sigma)
I'd certainly agree that the 4.1:1 final drive ratio is ideally suited to the 4/4. As for the ringing, have you tried a slightly thicker diff oil, would that help maybe? I can heartily recommend the Quaife ATB diff, but it's admittedly not a cheap fix for a ringing diff
