8 Speed Auto for HD trucks

Me Again wrote:

wilber1 wrote:

Me Again wrote:
With diesel engines wide flat torque curves, I just do not understand the need for more than 6 gears. Except for maybe RAM 68RFE that has tall 1st and 2 gears. Might be easier to just fix the ratios of the existing box than add more gears.

But how about two reverse gears, with a granny gears for the second one. Would be great for spotting trailers etc.

You want a gear that can give you max HP at WOT. More gears make it more likely you will have one instead of being in between gears and shifting back and forth as you go from a gear that doesn't give you enough power to one that puts you on the rev limiter and back again. Also 900 Lbft at 2000 RPM makes about 340 HP, at 2500 RPM it is closer to 428 HP. If you had an in between gear you could make 386 HP at 2250 RPM. So more gears can actually mean less shifting.

But why! I am towing at 24,500 combined without any issues, lack of power and/or starting issues!

So? What's your point? Torque to HP is a pretty simple calculation. I've towed the same weight with both four and six speed transmissions in diesels with flat torque curves from 1600 RPM to 2800 RPM. I would get stuck shifting back and forth between two ratios on hills quite often with the four speed. The six speed is much better but eight speeds would be even better still. I have a ZF 8 speed in my car and it is by far and away the best transmission I have ever had and always seems to be in the right gear.

CVT's aren't popular but one thing they can do is keep an engine operating at peak HP without getting stuck between two ratios.