Transmissions went from four speeds to eight, nine and ten within a generation. The gain was not acceleration; it was the ability to keep the engine barely turning at highway pace.
Engine speed drives fuel use at cruise
At a steady highway speed the engine is producing modest power, and much of the fuel consumed goes to internal friction and pumping losses rather than to moving the vehicle.
Those losses scale with how fast the engine is turning, so halving cruising rpm for the same road speed cuts a meaningful share of them.
An overdrive ratio, where the output shaft turns faster than the input, is how a transmission achieves that without changing the final drive.
American driving amplifies the benefit
Long interstate distances mean many vehicles spend hours in top gear at a constant speed, which is exactly the condition an overdrive ratio is designed for.
In dense urban driving the top gears are rarely reached, so the same hardware delivers much less benefit to a vehicle that never leaves city streets.
That distribution of use is a large part of why tall top gears became a standard American specification well before they were common elsewhere.
More ratios keep the engine in its efficient band
Adding gears does two things: it extends the spread between first and top, and it narrows the step between adjacent ratios.
Narrow steps let the transmission hold the engine near its most efficient operating point across a wider range of speeds rather than compromising between them.
The cost is complexity, weight and more frequent shifting, which is why calibration effort went into making those shifts imperceptible.
Torque converters and lockup matter as much
An automatic transmission's torque converter allows slip, and slip is wasted energy. A lockup clutch bypasses it once speeds are matched.
Extending lockup into lower gears and holding it during light acceleration captures efficiency that gear ratios alone cannot.
Modern converters lock very early and use controlled slip to absorb vibration, which is why a low-rpm cruise no longer produces the drivetrain shudder it once did.
The tradeoff shows up in throttle response
An engine turning slowly in a tall gear has little immediate power available, so any demand for acceleration requires a downshift.
How the transmission predicts that demand determines whether a pass feels prompt or hesitant, and it is one of the clearest differences between calibrations.
Continuously variable transmissions solve the ratio problem differently by having no fixed steps at all, which is why they appear where efficiency outranks driving feel.