Transmissions (and differentials) generally consume a percentage of the horsepower being transmitted through them, rather than a fixed amount like 40 HP.
There is a “brake-away” torque that you can measure. This is what it takes to get an input shaft to barely move from 0 rpm. It is a small fixed horsepower loss, and depends mostly on drag of oil seals and such.

The higher the gear reduction, the greater the percent of loss.
a 2.96 ratio consumes a lower percentage than 4.56 ratio

Straight cut gear teeth consume a lower percentage than beveled or helical gears, but straight cut makes more noise and can handle less peak power.

The “low hanging fruit” of automatic transmission loss was going from a simple torque converter to a lockup type.

Overdrive is a bit tricky,
Adding a set of overdrive gears INCREASES power lost to friction,
but shifting the engine to a lower rpm and raising intake manifold pressure has such a beneficial effect inside the gasoline engine that fuel economy of the vehicle improves.

A light loaded pickup traveling 70 mph with the typical 0.67 ratio overdrive and 3.55 ratio differential
would get a bit better fuel economy with
1.00 ratio transmission gear and differential ratio 2.37 (.67 x 3.55)
but would get worse fuel economy towing a rated weight heavy trailer
using its third gear ratio 1.3 with that 2.37 diff,
instead of 1.00 gear and 3.55 diff.

The 98% efficiency numbers of modern “automated manual transmissions” kind of stun me.
I wonder: is that just in 1:1 top gear at some low rpm? Best case advertising hype that a vehicle seldom runs near?