https://www.fueleconomy.gov/feg/Find.do?action=sbs&id=34210&...
Did I stutter?
These 8-10 speed transmissions absolutely make a huge difference. Just one example I'm familiar with is the Dodge charger. The 2014 model had a 5 speed automatic, and the 2015 had an 8 speed. Otherwise they are exactly the same car. The 8 speed transmission resulted in a 3MPG improvement on the highway from 27MPG to 30MPG. That is a huge improvement for only changing literally one component on the car.
Sure it's "only one component" but you 5/3rd'ed the number of moving parts in it. And "it" is a component that frequently limits the effective useful life of the car. I said that tradeoff didn't make sense until the past decade or so. An example of someone in 2015 choosing to pick up ~10% fuel economy in something that was already pushing high 20s is, to put it undeservingly charitably, not an example to the contrary.
The ZF 8 Speed Automatic actually has less moving parts than the ZF 6 speed automatic it replaced.... And the it has proven to be one of the most reliable automatic transmissions produced. The old 4, and 5 speed transmissions are notoriously unreliable.
"They don't make a "big" difference. If they made a "big" difference you'd have seen them decades ago. We've had the tech for as long as we've had automatic transmissions. More gears is just more stacked planetary gearsets."
I guess I just don't understand your argument. Are you saying these new transmissions don't make a difference because they didn't do it earlier?
Even though fuel economy testing proves they clearly do make a difference....
And these new transmissions most certainly are not "just more stacked planetary gearsets". There has been decades of technology and efficiency improvements integrated into these new transmissions. I'm quite certain the transmission control module and shift programming, clutch packs and materials, etc. are different than an old 4 speed transmission.
The problem is that the ECU itself is going to be much better at optimizing the engine load, rpm and gear than someone driving a manual even with a gear shift hint on something like the EPA tests. And AFAIK the epa highway test is more like a boulevard test than a test of efficiency at a constant say 65 MPH, so gear shifting itself will cause a loss that won't be made up by the slightly higher efficiency of the manual itself since locking torq converters/etc. Thats part of the reason why people have been picking automatics, they started getting better EPA ratings, what 20 years ago when they still had roughly the same number of gears, and not long after started posting better 0-60 numbers as well.
Plus, car manufactures frequently do things like change the default tire with a given package (the car you linked as differing gas tanks too). So you get the sporty version and along with the larger engine you get fatter less efficient tires.
What I'm trying to say, is that you can't claim the 3MPG diff on that car is entirely just the number of gears by itself.
Unfortunately I'm not aware of any car that this would be possible to test. I picked the Dodge Charger because it's the closest option we have, and very little changed between those two model years except the transmission.
I participated in forums back in the ~2015 era because I had a Dodge Challenger at that time. And it was quite clear that the 8 speed made a huge improvement in fuel economy in the 2015+ cars. That matches the official EPA tests.
https://mechanics.stackexchange.com/questions/2161/fuel-effi...
Note that the most efficient curve for a given load is frequently ~1k rpm. Which given the usual gear ratios a 5 speed transmission should be more than capable of keeping it a couple hundred RPM on one side or the other of the actual peak.
Its more likely a long list of optimizations where the most noticeable was the gear count change on a car that probably wasn't particularly optimized to begin with. A good part of it might have been the transmission/torque converter, but even then it had to be a pretty crummy transmission for that large of an improvement. The usual things to look for are drag improvements, weight improvements, tire efficiency, oil/transmission viscosity changes, and an untold number of small transmission and engine tuning changes to the ECUs as well as any number of subtle mechanical changes to the intake and exhaust to ignition system changes. Most of those things aren't going to show up on a white paper at the dealership.
So while the 0W-30 oils and the like have an effect, the tires things as I mentioned is huge on some cars. You can walk around in the truck area of many dealerships and see 10% deltas in MPG on some of the them that are identical except for things like offroad tires and 300Lbs of bed liner.
Ex: https://media.ford.com/content/dam/fordmedia/North%20America...
Note that the top end gearing is pretty similar between the manual and automatics. There is a larger difference in 1st gear, which implies that the automatics might launch a bit faster, although I'm no expert it might just be compensating for the lack of being able to dump the clutch.
Also, it is possible to build manuals with more gears, it just isn't usually done for weight and ergonomic issues unless your driving something like an 18 wheeler.
Except, most automatics are going up past 5th gear. 7, 8, even 10 speed auto's are pretty common. Those higher "highway" gears make a big difference in terms of engine revs (which equals less noise and better MPG).