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?
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.