Why Are Electric Vehicle Speedometer Round?

Sep 10,2021

The radial layout of the electric vehicle speedometer is a holdover from when it was a mechanical device. The typical speedo of the mid-1900’s transmits rotational speed from gearing in the transmission to the needle in the dashboard by way of a flexible, springy wire cable. The cable is sheathed in a jacket and squared off at each end so it can be spun by the transmission and thus spin a magnet where it terminates behind the instrument panel. That orange needle itself is not physically connected to any rotating parts in the drivetrain. The needle is held in place by a lightweight spring pushing it toward the zero position.

Behind the panel, the needle is attached to a metal “speedcup.” The magnet attached to the cable generates a magnetic field that grows stronger the faster it spins. This magnetic field acts upon the speedcup, providing the force that pushes the needle away from zero. So, this mechanical design was the most reliable, cheapest way to translate the rotation of circular wheels (driven by the transmission) to the rotation of a needle around a circular gauge. “Circle” is the native language of the car.. at least for the drivetrain. Additional translation from circular rotation to the linear movement of a mercury thermometer, for example, would have added cost and complexity to the mechanical design. When cars had lower top speeds, there were some dashboards with more linear instrumentation layouts, but they achieved that by shorter travel of a longer needle along the arc of a bigger circle. True linear movement speed gauges are very rare.

Gregor noted that his speedo had a non-uniform scale. Well, check out this old Cadillac; the scale is stretched out on both ends.

However, in the 1980’s, there was an eletronical revolution that rendered the cable and magnet setup unnecessary; it was replaced with electromagnetic speed sensors, electronically controlled needles, and sometimes digital numeric displays. Fast-forward to 2012 where we now have almost complete control over the instrument panel layout and can project graphics onto Heads-Up Displays (HUDs) that seem to hover in space in front of the driver. The cluster below from the latest BMW 5 series is a 10.25″ LCD screen that displays distinct graphics depending on the selected driving mode. Two modes are shown below; ECO PRO and SPORT. Note that in SPORT mode with manual shifts activated, the two main gauges are connected by a red glow to signify the important relationship between them, as I described above.

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