How do early transmissions know when to shift? How do they go to the right gears for the right conditions (heavy acceleration, going up a hill etc.)? I cannot imagine how it is done without electrical components, but it seems like they were purely using hydraulics.

  • As an interesting source check out Lanchester...
    – Solar Mike
    Commented Sep 20, 2019 at 16:43
  • 1
    Yes, with hydraulics. Here is a description of how it used to be done. Commented Sep 20, 2019 at 17:17
  • @WeatherVane Thank you! The link is really helpful!
    – Eric x
    Commented Sep 20, 2019 at 17:37
  • The governor controls shift points.
    – Moab
    Commented Sep 21, 2019 at 19:02
  • I think I'm confused. Have you meant "purely mechanical" as in "without electronics" or "purely mechanical" as in "only gears shifting, no torque converter". Because torque converter doesn't shift at all, it's continuous.
    – Agent_L
    Commented Mar 3, 2021 at 13:49

2 Answers 2


//edit: This answer is based on assumption that "purely mechanical" excludes torque converters and we're talking about actual gears shifting, as in "manual gearbox with a mind of it's own".

RPM alone.

When RPM goes beyond certain range, it's a cue to shift up. When RPM drops below another threshold, to downshift. That's all.

The whole point of a gearbox is to maintain engine RPM in optimal range. All other things are just tweaking this match in anticipation of a future change Eg. going uphill you're mentioning is merely a prediction that the speed of the vehicle won't actually increase enough to make the most out of upshift. By applying this prediction you defer shifting - but disregarding the condition and shifting as usual will not make the gearbox unusable, it'll just have to shift down again soon.

Also keep in mind that the bigger the engine, the more the power margin the system as a whole has. So while a tiny, overworked engine needs a lot of skillful shifting to stay in this one tiny rpm spot where it can manage, a big, overpowered engine will easily cover for any shifting mistakes. (That's the original reason why automatics are ubiquitous in USA but unpopular in Europe: US engines are bigger, so the gearbox has so much less work to do.)

Also, as some early mechanical automatics still depended on driver pressing the clutch, all the sophisticated decision making on "when to shift" was still done by a protein-based electrical computer. The gearbox only had to decide if and which way to shift, which is a very simple decision to make based on - wait for it - RPM alone.

  • I wouldn't say point of a gearbox is to maintan engine RPM in an optimal range. In my opinion, the point of a gearbox is to have 99% of the time the most economical gear, while providing high RPM for 1% of the time when needing to accelerate fast. It's not thus only RPM, it's also the throttle position that matters.
    – juhist
    Commented Oct 16, 2020 at 15:21
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    There is no such thing as "economical gear". There is only "economical RPM", and the proper gear is one that matches vehicle speed with this RPM. The "need to accelerate fast" is done by notching into higher RPM, with more power but even more fuel usage. But it's still "keep shifting until desired RPM reached", just the target RPM is different.
    – Agent_L
    Commented Oct 17, 2020 at 16:27

Hydraulics or vacuum. Engine spins torque converter and pump, fluid flow directed by valve body for shifting.

  • The first automatic transmission was GM's Hydramatic and I don't think that hydraulics are strictly categorized as 'mechanical' but they are 'non-electric'.
    – user16128
    Commented Feb 18, 2020 at 2:20

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