A car's speed and RPM indicators usually start at 7, lower left, but airplanes' altitude and airspeed indicators start at noon. What's the origin of this convention?

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    $\begingroup$ Aircraft RPM meters typically start the same place the automotive ones do. VSI typically points either at 9 o'clock or 3 o'clock. Many gauges start at 10 o'clock to 2 o'clock (temperature, oil pressure)... $\endgroup$ – Ron Beyer Mar 7 at 20:44
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    $\begingroup$ In (motor)gliders you'll often find the airspeed indicator resting at 6 $\endgroup$ – Gypaets Mar 8 at 8:12
  • $\begingroup$ Aircraft indicators don't like to get out of bed till the crack of noon. $\endgroup$ – Robusto Mar 9 at 18:14

Most old-style indicators (RPM, cyl. head temperature, outside air temperature, oil pressure, etc.) are geared in such a way as to present their full operating range over a needle swing of 270 degrees. It is also common for the gauge mechanism to be oriented in such a manner as to put the nominal operating point somewhere near the middle of the needle's swing arc, where the gauge mechanism introduces the fewest errors in the reading.

Early (crude!) altimeters were also 270 degree swing gauges (i.e., barometers), but this made it impossible for the pilot to read that gauge to an accuracy better than perhaps 500 feet- which is insufficient for modern use. Later versions of the altimeter had gear trains inside them to magnify the needle movement enough to read the gauge to an accuracy of ~50 feet, but this in turn dictated that the gauge range cover multiple revolutions of the needle around the face of the instrument... like a clock.

Since we are all used to reading clocks that start at the "12:00" position, the multi-turn altimeters were set to start at 12:00 too and geared so one full needle revolution corresponded to one thousand feet on the "minute" needle, and ten thousand feet on the "hour" needle. This worked fine for small planes that rarely flew higher than 10,000 feet.

Registering multiples of 10,000 could be done with a third needle but a three-needle gauge isn't the easiest thing in the world to quickly scan. A better solution is to have a window cut into the gauge face behind which altitudes greater than 10,000 feet are displayed as numbers instead.

As an aside, in older American-made car speedos (1960's-era), 60 mph was most commonly positioned at the 50% scale point ("straight up" on the dial face) to give the driver a quick visual indication of 60, without having to read the numbers. When the US federal maximum speed limit was reduced to 55 during the oil crisis of the mid-1970's, 55 was then positioned at the straight up spot.

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    $\begingroup$ Also, older automotive speedometers weren't circular; the needle traversed from left to right, roughly from what would be 11 o'clock to 1 o'clock, with neither the top arc nor the bottom 2/3 of such a dial being present. When you consider the height of automotive instrument panels, that flattened display makes sense -- less real estate to work with. As that wraps into a circular gauge (ask the automotive historians why cars went to round dials), the zero becomes left and low (7 to 8-o'clock ish), and the max becomes right and approaching horizontal (2 to 3 o'clock ish). $\endgroup$ – Ralph J Mar 7 at 22:52
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    $\begingroup$ There is a rationale to scale and orient "health" gauges (temperatures, pressures, RPMs, etc.) such that the pointers all align in the same direction, such as 3 o'clock, when values are in their "green" or nominal range. This can help make an off-nominal condition stand out more in a quick visual scan. It can put zero and maximum values at seemingly odd positions on the dial, but that is functionally unimportant. $\endgroup$ – Anthony X Mar 8 at 3:41
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    $\begingroup$ "Federal" speed limit refers to which country? Quite a few countries have federated government, but you didn't say which one you're talking about. (Admittedly, not many places use mph, but even so...) $\endgroup$ – Toby Speight Mar 8 at 13:08
  • $\begingroup$ +1, but when I was flying gliders in a metric country most altimeters had zero at 6 o'clock. $\endgroup$ – Martin Argerami Mar 8 at 13:27
  • $\begingroup$ As for what could be done, most "modern" analogue readouts are now assymetric, the first 90 deg covers less range than the next 90 deg. Or for the barometer types they are usually separated into "OK" range and "NOT OK" - which each can have a separate sensitivity. That is at least top of the line where I work, for the odd and analogue things where the computer doesn't take care of it all. $\endgroup$ – Stian Yttervik Mar 8 at 15:01

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