The more gears the better. An engine only has one specific rpm where it is running at peak efficiency/power for the situation, the closer it can stay to that number, the better. The only way to stay close to that number, is with a lot of gears, or even better, a computer controlled CVT. I'm amazed that more auto manufacturers haven't gone to computer controlled CVT, they are absolutely the way to go.
A few people have mentioned truck transmissions, they show the difference even more clearly. A lot of people are getting away from manual transmissions in trucks and going to automatics for that very reason and have been getting amazing results. The Allison automatics are blowing away every manual transmission out there in big trucks.
I work in heavy haul and move up to around 250k loads, my truck dyno's 741hp to the ground, has an 18 speed transmission, a 4 speed auxiliary transmission, and 2 speed rear ends, that's 144 possible forward gear combinations. Now there's no way to use all of them in a row, but I have an option for pretty much any situation. Even with all that, our new truck with a 600hp engine (that should be 500ish to the ground) and an Allison 6 speed automatic absolutely blows mine away in every category. It gets 1mpg better fuel mileage, 50% longer drive tire life, a drastic reduction in drivetrain maintenance costs, and with an even load on both trucks, it absolutely blows mine away despite having about 240 less horsepower.
The 1 mpg might not sound like much, but it's actually a 20% increase in mileage, which saves us about $8k a year in fuel costs A full set of drive tires cost about $8k a set and at a set per year that's another $4k savings. As far as drivetrain maintenance, we are seeing absolutely no wear in the carrier bearings, U joints, power divider, or pinions, the diffs themselves also run noticeably different. My truck on the other hand is already due for new U joints and carrier bearings even though they were changed at about the same time as we got the truck with the automatic, that's another thousand in maintenance costs, plus half a day of down time.
So the automatic saves us between 12 and 15 thousand dollars a year right off the bat, and outruns trucks with far more horsepower. The reason for pretty much all of this is that the Allison automatic never lets off the throttle to shift. It grabs the next year under full power and never leaves the drivetrain coasting while it changes gears. It also uses its torque converter to mimick more gears in between the 6 that it has. Instead of jumping straight from one gear to the next, it slowly ramps up the ratio until the next gear is fully engaged. The manual transmissions, and the Eaton automatics, which are an automatically shifted manual transmission, have to let off the throttle, allowing the truck to coast and lose momentum, grab the next gear, then power back into the throttle to accelerate again. It is quite literally two steps forward and one step back and wastes a huge amount of energy.
There has been quite a bit of talk of going to a CVT transmission for trucks, which I think would be great. It makes nothing but good sense to put the engine at its peak power/efficiency rpm (which varies with the load placed on it) and let it already stay at that number while the power is put smoothly, evenly, and uninterrupted to the ground.
So to answer the question of how many gears is enough?? Infinity is my answer. There isn't enough gears out there to let an engine truly work correctly.
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