Generally, a redline upshift on a 6-speed already places the RPM drop well within the optimal power band of most engines except the peakiest. In fact, for some engines one can argue that a 5-speed already does that. There is no real performance advantage. In fact, shifting too many times can actually make a car slower -- slippage in the clutch packs and/or within the torque converter during any shift means that momentarily some power is used to make heat in the transmission fluid instead of accelerating the car.

The main benefits the latest trannies bring is largely in fuel economy. The 8-speed XF Supercharged is 2 MPG better in EPA test cycles on the Highway than the 6-speed equipped cars (City numbers are essentially unchanged) And what is most needed and most beneficial is ratio spread not number of gears. The ZF 8HP familt has a 7.05:1 ratio spread between the tallest and lowest gears. This is considerably wider than a 6HP which is 6:1. This allows the top gear cruising rpm to be lower while keeping the 1st gear similar in ratio so performance is unaffected.

One can easily make a case for the fact that a 6-speed box with an increased ratio spread would be just as good -- or in certain cases better. But it is more difficult to make a 6-speed with a 7:1 spread than a 7 or 8-speed with that spread because you'll have to do it with fewer planetaries. Fewer gears in in succession means that each platnetary set has to be larger in diameter. This is problematic since a fat transmission is intrusive of the footwell in F-R cars. This coupled with the advantageous of being able to claim more speeds in "marketing" activities lead most makers down the road of ever increasing # of gears. They do that, then come back and skip shift the transmission in normal driving and/or do things like starting in 2nd to get around having too many speeds and is optimal in certain driving conditions!

 

Dwight Looi,

You seem to know a little bit about trannies or at least the tranny in the Jag, lol.